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REINTRODUCING JAVAN GIBBONS (HYLOBATES MOLOCH): AN
ASSESSMENT OF BEHAVIORAL PREPAREDNESS
_______________
A Thesis
Presented to the
Faculty of
San Diego State University
_______________
In Partial Fulfillment
of the Requirements for the Degree
Master of Arts
in
Anthropology
_______________
by
Jaima Hillary Smith
Spring 2011
iii
Copyright © 2011
by
Jaima Hillary Smith
All Rights Reserved
iv
ABSTRACT OF THE THESIS
Reintroducing Javan Gibbons (Hylobates moloch): An Assessment of Behavioral Preparedness
by Jaima Hillary Smith
Master of Arts in Anthropology San Diego State University, 2011
The purpose of this thesis is to contribute knowledge to the reintroduction program
for Javan gibbons (Hylobates moloch) in West Java, Indonesia in an effort to improve the conservation of the species in the wild. Reintroduction programs provide an opportunity for animals that have lived in captivity to have another chance at living in the wild, and may be one way of reestablishing populations that have become locally extinct. It is crucial that release candidates either maintain or develop the appropriate behavioral repertoire necessary for living in the wild, and have undergone an extensive period of rehabilitation. My research goal was to examine whether select pairs of Javan gibbons living at the Javan Gibbon Rescue and Rehabilitation Center (JGC) in Gunung Gede-Pangrango National Park, Java, Indonesia exhibit the appropriate behaviors necessary for survival before they are released back into the wild. Behavioral data were collected from July to November 2009, yielding a total of 337 hours. I conducted pre- release behavioral observations on three pairs of Javan gibbons that were deemed potential candidates for release by the staff of JGC. I observed each individual gibbon utilizing focal animal time-interval sampling and ad libitum as necessary. I also conducted post-release observations on the first pair of Javan gibbons released in October 2009. Pre-release results indicate the pairs of Javan gibbons in this study did not satisfy all of the suggested behavioral criteria. However, the behavior that did not match the recommendations (time spent in positive pair association) will not necessarily hinder the gibbons’ survival when they are released. It is recommended that rehabilitated gibbons spend at least 7% of total time active engaged in positive association behavior (i.e., groom, play, and copulation); not one of the pairs of Javan gibbons in this study met that criteria. This specific criterion was determined to not be a valid measure with regards to pair-bond cohesiveness, because it fails to include maintenance of close-proximity - one of the primary social interactions for adult gibbons. In addition, the other behavioral criteria the gibbons did satisfy (i.e., effectively brachiating, remaining in upper level of enclosure, feeding/foraging) are those that will be most beneficial for their survival in the wild. Post-release observations indicate that the gibbon pair was successful at locating and procuring the appropriate food sources. They jointly defended their territory against sympatric monkeys, and they remained in the upper level of the canopy. Most importantly, the pair remained together after release and the female regularly vocalized. However, the gibbons exhibited a high level of aggression towards human observers. The results from this thesis can inform a revision of the proposed recommendations for evaluating the behavioral preparedness of rehabilitated gibbons. Based on the overall behavioral profile of the released gibbons, it would seem that at least some Javan gibbons are capable of acquiring the necessary survival skills to live in
v
the wild after a period of rehabilitation. At the same time, considering the aggressive behavior that the gibbons displayed towards humans, caution must be taken for future observation and monitoring of reintroduced Javan gibbons and future release sites should have very minimal human activity.
vi
TABLE OF CONTENTS
PAGE
ABSTRACT ............................................................................................................................. iv
LIST OF TABLES ................................................................................................................. viii
LIST OF FIGURES ................................................................................................................. ix
ACKNOWLEDGEMENTS .......................................................................................................x
CHAPTER
1 INTRODUCTION .........................................................................................................1
2 LITERATURE REVIEW ..............................................................................................5
Gibbon Behavior and Ecology .................................................................................5
Threats to the Survival of Javan Gibbons ................................................................9
Loss of Habitat/Habitat Fragmentation ..............................................................9
Illegal Pet Trade ...............................................................................................13
Small Population Processes .............................................................................15
Rehabilitation and Reintroduction ...................................................................19
Gibbon Rescue and Rehabilitation Centers ...........................................................23
Gibbon Rehabilitation Project (GRP) ....................................................................23
Kalaweit Gibbon Center (KP) ................................................................................24
The Javan Gibbon Rescue and Rehabilitation Center (JGC) .................................25
3 RESEARCH QUESTION AND METHODS ..............................................................27
Research Question .................................................................................................27
Methods..................................................................................................................27
Study Site .........................................................................................................27
Study Group .....................................................................................................28
Data Collection ......................................................................................................30
Pre-Release ......................................................................................................31
Post-Release .....................................................................................................31
Data Analysis .........................................................................................................34
4 RESULTS ....................................................................................................................40
vii
Between-Site Comparison .....................................................................................40
Within-Site Comparisons .......................................................................................41
Activity Budget ................................................................................................41
Social Behavior ................................................................................................42
Stereotypic Behavior ........................................................................................43
Human-Directed Behavior ...............................................................................44
Maintenance of Proximity................................................................................44
Location in Enclosure ......................................................................................45
Locomotion ......................................................................................................47
Evaluation of Recommended Behavioral Criteria ...........................................49
Release of Javan Gibbon Pair ................................................................................49
5 DISCUSSION ..............................................................................................................54
Behavioral Patterns ................................................................................................54
Social Behavior ......................................................................................................56
Stereotypic Behavior ..............................................................................................58
Human-Directed Behavior .....................................................................................58
Location in Enclosure ............................................................................................60
Locomotion ............................................................................................................61
Conservation Implications .....................................................................................61
Evaluation of Behavioral Criteria ..........................................................................63
Lessons Learned from the Release ........................................................................66
Local Initiatives and Support .................................................................................70
REFERENCES ........................................................................................................................72
viii
LIST OF TABLES
PAGE
Table 1. Description and Pair Names of the Study Gibbons ...................................................31
Table 2. Ethogram for Javan Gibbon .......................................................................................32
Table 3. Proposed Criteria for Assessing the Suitability of Gibbons for Release ...................35
Table 4. Summary of Behavioral Criteria for Pre-Release Observations ................................49
Table 5. Revised Behavioral Checklist ....................................................................................67
ix
LIST OF FIGURES
PAGE
Figure 1. Map illustrating the distribution of the western and central population of Javan gibbons according to Andayani, N., Morales, J. C., Forstner, M. R. J., Supriatna, J., & Melnick, D. J. (2001). Genetic variability in mtDNA of the silvery gibbon: Implications for the conservation of a critically endangered species. Conservation Biology, 15, 770-775. Darkened areas represent forest areas where gibbons are still present. Outlined areas represent potential suitable habitat, but are uninhabited by wild gibbons. Kappeler (Kappeler, M. (1981). The Javan silvery gibbon (Hylobates lar moloch). (Doctoral dissertation) Universitat Basel, Basel, Switzerland.) surveyed Gunung Salak and found no gibbons. ..................................................................................................17
Figure 2. Map illustrating the location of JGC. ......................................................................26
Figure 3. Naturalistic enclosure at Javan Gibbon Rescue and Rehabilitation Center. ............29
Figure 4. Release site, Patiwel. ................................................................................................30
Figure 5. Septa and Echi being released from their enclosure at (Photo by Duhe Anfield). .......................................................................................................................33
Figure 6. Septa after release (Photo by Duhe Anfield). ...........................................................34
Figure 7. Average percent of observation records gibbons spent in activity at JGC, KP, and CALS. Data represent combined percentage of male and female gibbons in each activity. JGC-6 individuals (3 adult males; 3 adult females); CALS-5 individuals (3 adult males; 2 adult females); and KP-41 individuals (number of each sex is unknown). ...............................................................................41
Figure 8. Percent of observation records the gibbons spent in different activities. ................42
Figure 9. Percent of observation records gibbons spent in proximity. ....................................45
Figure 10. Percent of observation records spent in location of enclosure. ..............................46
Figure 11. Percent of observation records gibbons spent in different modes of locomotion. ..................................................................................................................47
Figure 12. Percent of observation records gibbons spent in different modes of locomotion. ..................................................................................................................48
Figure 13. Percent of observation records gibbons spent in different modes of locomotion. ..................................................................................................................48
Figure 14. Septa and Echi post-release (Photo by Anton Ario/CI-Indonesia). .......................50
Figure 15. Septa and Echi post-release (Photo by Anton Ario/CI-Indonesia). ........................68
x
ACKNOWLEDGEMENTS
I would like to sincerely thank Anton Ario and all of the staff at the Javan Gibbon
Rescue and Rehabilitation Center, Conservation International-Indonesia, and Taman
Nasional Gunung Gede-Pangrango for providing me with the opportunity to conduct my
fieldwork at JGC and for always being so warm and welcoming to me. Anda sekalian sangat
istimewa teman-teman kepada saya. I would like to personally thank Dr. Jatna Supriatna for
sponsoring my fieldwork at JGC and Ermayanti and Jarot Arisona for guiding me through the
research permit process and helping me to extend my stay in West Java. I would like to
thank the Indonesian government for giving me permission to conduct research in West Java.
I am very grateful to Ibu Yetti at RISTEK and Ibu Yani at Imigrasi Jakarta Selatan for all of
their help and friendship during my stay in Indonesia. I thank Drs. Erin P. Riley, Arion
Mayes, and Rulon Clark for serving on my thesis committee and providing me with helpful
comments. In addition, I would like to thank Dr. Erin P. Riley for all of her support,
guidance, and friendship. Also, Dr. Nicholas Malone provided me with invaluable advice
and support throughout my research, thank you. I am grateful to Alan Mootnick for all of his
support through the years and for providing me with the opportunity to first visit JGC during
the summer of 2008. Thank you to Dan Levinson, Leigh Fitzgerald, and Jenny Minitti-
Shippey for taking such wonderful care of my Luna while I was in Indonesia. A very special
and loving thank you to my father, Robert Otis Smith, for funding my research and for
always being so supportive. My dear mother Lana Relation, thank you for all of your love
and support. Sorry you were always so worried I was going to be attacked by a Javan
leopard.
1
CHAPTER 1
INTRODUCTION
It is estimated that almost half of the world’s nonhuman primates are threatened with
extinction and 29 percent are categorized as Endangered or Critically Endangered
(International Union for Conservation of Nature and Natural Resources [IUCN], 2010).
Current conservation efforts seek to preserve the diversity of the Primate Order and ensure
the survival of representative populations of species in their natural habitats (Southwick &
Blood, 1979). In order for these efforts for primates to be successful, as well as for other
wildlife, it is crucial that the cause of the threats faced by the different species in their
habitats are thoroughly understood (Nijman, Yang Martinez, & Shepherd, 2009). The
primary extrinsic threats to the majority of primates are undoubtedly loss of habitat and
hunting. In addition, as remaining populations continue to decrease, the effects of secondary
threats, such as the use of primates in traditional medicine and the illegal pet trade, become
more important (Cowlishaw & Dunbar 2000; Nijman et al., 2009). The illegal trade of
primates can have a drastic effect on the last surviving populations of already rare species.
Young primates are removed from the wild and most often, reproductively viable adults are
killed in the process. In addition, as a result of the illegal trade network, primates are
becoming displaced and orphaned, which brings them into various types of rescue and
rehabilitation centers (Cheyne, 2004).
Rehabilitation and reintroduction programs have been widely used as a method of
conservation for endangered species for some time (Kleiman, 1989). Reintroduction
programs provide an opportunity for animals that have lived in captivity to have another
chance at living in the wild, and may be one way of reestablishing populations that have
become locally extinct (Komdeur & Deerenberg, 1997). Reintroduction addresses
conservation on two different levels. First, animals that are kept illegally as pets are rescued,
rehabilitated, and then returned to the wild; and second, by reintroducing animals into areas
where they are locally extinct, the wild populations are supplemented and potentially more
forest can be protected (Cheyne, 2006; Kleiman, 1989). It has been acknowledged that
2
preservation of wild populations and their habitats is imperative for the conservation of many
wild species, and rehabilitation and reintroduction may play a significant role in supporting
the wild populations, as well as raising awareness of the plight of these species (Cheyne,
Chivers, & Sugardjito, 2008).
All species of gibbons (Hylobatidae) have been in decline over the past 30-40 years,
primarily due to loss of habitat and the subsequent fragmentation of forests through timber
felling, charcoal burning, encroachment cultivation, rubber plantations, and tea and pine
plantations (Cheyne, 2004; Nijman & van Balen, 1998). Other factors contributing to their
demise include the illegal wildlife trade, hunting, and the use of their body parts in traditional
medicine (Cheyne, 2004). Rehabilitation and reintroduction of gibbons is a relatively new
conservation strategy. There are very few data published on the successful reintroduction of
gibbons back into their native habitat. With 15 of the 17 gibbon species currently listed as
Endangered or Critically Endangered (IUCN, 2010; Van Ngoc et al., 2010), the future of all
gibbons is truly in peril (Cheyne et al., 2008). Many gibbons are kept illegally as pets and a
large number of these end up in rescue and rehabilitation centers when the owners realize
they can no longer control the gibbon. The reintroduction of former pet gibbons may be one
way to ensure the survival of many of these threatened species of gibbons in the wild
(Cheyne et al., 2008).
The Javan Gibbon Rescue and Rehabilitation Center (JGC) was established in 2003
and is a collaborative effort between Conservation International-Indonesia, the Silvery
Gibbon Project, University of Indonesia, and the Indonesian Ministry of Forestry. The center
receives donated or confiscated pet Javan gibbons (Hylobates moloch) with the goal of
restoring their physical and psychological health back to as high a level as possible. This
process includes assessing their medical and behavioral status and placing them into an
appropriate social environment of an adult male and female (Supriatna, 2006). In the wild,
Javan gibbons generally live in two-adult group units consisting of an adult male and female
and their dependent offspring (Fuentes, 1999, 2000; Palombit, 1994; but see Malone, 2007).
Therefore, this is the preferred social environment for gibbons at the center and it is
recommended that the gibbons are released in pairs. By releasing gibbons in established
pairs, this will hopefully ease the transition from captivity to the wild and eliminates the need
for the gibbon to have to find a mate and establish a territory when released (Cheyne, 2004).
3
In addition to the rehabilitation process, JGC also conducts non-invasive behavioral research
and provides public awareness and conservation education programs that focus on the
preservation of the Javan gibbon (Supriatna, 2006). The ultimate goal of JGC is to release
rehabilitated Javan gibbons back into the wild.
The objective of this thesis project was to contribute to the Javan Gibbon Rescue and
Rehabilitation Center’s goal of rehabilitating and reintroducing Javan gibbons back into their
native habitat. The study consisted of conducting behavioral observations on pairs of Javan
gibbons living at JGC in order to determine if they were suitable candidates for release.
Gibbons that have been removed from the wild at a young age and have lived as pets for
most of their adult lives face a variety of challenges in the rehabilitation process. These
challenges include: learning how to vocalize in the appropriate social context, appropriately
socializing with conspecifics (Mootnick & Nadler, 1997), overcoming the potential
prevalence of stereotypic or human-directed behaviors, learning how to locomote efficiently
(i.e., brachiate, developing balance), and learning how to locate and procure food (Cheyne,
2004; Kleiman, 1989). It is imperative to have an understanding of gibbon behavior during
both the pre-and post-release phases of the rehabilitation process, in order to help ensure the
gibbons will have the best opportunity to adapt to a life free from human dependency in the
wild. Many of the earlier reintroduction projects with lar gibbons (Hylobates lar) in
Thailand were unsuccessful because of inadequate planning throughout the rehabilitation
process and a lack of knowledge pertaining to the behavior and ecology of the species
(Cheyne, 2004; Clemmons & Buchholz, 1997).
Behavioral studies can contribute a significant amount of knowledge to reintroduction
programs by examining the underlying evolutionary mechanisms of species’ behavior, such
as the driving forces behind social and spatial organization, and by developing an
understanding of how they respond to different social and environmental conditions
(Clemmons & Buchholz, 1997). The rehabilitation process will vary depending on the
species. Species that spend an extended period of time learning valuable social and life skills
from their parents, such as monkeys and apes, may prove more challenging in the
rehabilitation/reintroduction process (Cheyne, 2004). As Cheyne (2004) demonstrated in her
work at the Kalaweit gibbon center in Central Kalimantan, Indonesia, during the
rehabilitation process it is crucial to document the behavior of gibbons in captivity prior to
4
their release, so that we are able to compare the pre- and post-release behavior as a way to
determine the relative success of the reintroduction project. Therefore, monitoring the
gibbons’ behavior before they are released is a crucial component in the reintroduction
program for the Javan gibbon and hopefully will ensure a successful release.
In addition to fully documenting pre-release behavior of the gibbons, it is critical to
monitor them for an extended period of time after they are released and their behavior should
be compared to wild conspecifics. There are very few data available on the behavior of
reintroduced gibbons and their ability to adapt to a new environment (Cheyne, 2004;
Kleiman, 1989). Cheyne (2004) presents the only comprehensive study of the rehabilitation
process and reintroduction of white-bearded gibbons (Hylobates albibarbis) and Mueller’s
gibbons (Hylobates muelleri) to date. This study constitutes the first step at developing a
more systematic approach to the rehabilitation process at JGC by fully documenting the
gibbons’ behavior both pre- and post-release. Considering the behavioral ecological data on
wild Javan gibbons is extremely limited (Kappeler, 1981; Malone, 2007), if the
reintroduction project is to persist as a conservation mechanism for Javan gibbons, we must
also engage in further studies on their behavior and ecology in the wild to increase our
knowledge.
5
CHAPTER 2
LITERATURE REVIEW
The literature review comprises a discussion of research relevant to this thesis
including: gibbon behavior and ecology, threats to the survival of Javan gibbons, and
rehabilitation and reintroduction programs.
GIBBON BEHAVIOR AND ECOLOGY
Gibbons, whose family name, Hylobatidae, is Greek for “dweller in the trees”, are the
smallest of the apes and are distributed throughout tropical and subtropical forests of South,
East, and Southeast Asia. Their range extends from northeastern India to southern China,
Bangladesh, Myanmar, Vietnam, Lao PDR, Cambodia, the Malay Peninsula, Java, Borneo,
Sumatra, and the Mentawai Islands (Cheyne, 2004; Malone, 2007). In terms of numbers of
species and individuals, gibbons are the most successful and widely distributed of all the
extant apes (Cheyne, 2004). Nonetheless, they are threatened throughout their range
primarily due to loss of habitat, the illegal pet trade, use in traditional medicine, and poaching
all contribute to their decline. All gibbon species are listed on Convention on International
Trade of Endangered Species (CITES) and hold various positions on the World Conservation
Union (IUCN, 2010) Red List (Cheyne, 2004).
The Hylobatidae have been divided into four different genera based on their number
of chromosomes, as well as morphological, vocal, and behavioral differences that exist
throughout the family. The four genera are: Hoolock, Hylobates, Nomascus, and
Symphalangus (Bartlett, 2007; Malone, 2007; Mootnick & Groves, 2005; Roos &
Geissmann, 2001). There are currently 17 different species recognized in the Hylobatidae
(Van Ngoc et al., 2010). The focus species of my study is the silvery or Javan gibbon. It is
endemic to the island of Java and is considered one of the rarest hylobatids (IUCN, 2010;
Supriatna, 2006).
The Javan gibbon is currently listed as Endangered on the IUCN Red List of
Threatened Species (IUCN, 2010) based on the 2001 criteria C2a(i), which refers to a
6
“population estimated to number less than 2500 mature individuals and a continuing decline,
observed, projected, or inferred, in numbers of mature individuals and no subpopulation
estimated to contain more than 250 mature individuals.” Currently, it is estimated that 4,000
to 4,500 Javan gibbons remain in the fragmented forests of Java (Supriatna, Mootnick, &
Andayani, 2010). The threats to the survival of Javan gibbons are loss of habitat/habitat
fragmentation, small population processes, and the illegal pet trade. The Javan gibbon was
among the first species to become protected by Indonesian law in 1925, and is listed in
Appendix I of CITES, which prohibits all international trade of the species, its parts, and
derivatives (Geissman & Nijman, 2006; Nijman, 2006). There are possibly two recognized
subspecies, Hylobates moloch pongoalsoni and Hylobates moloch moloch (Andayani et al.,
2001). The Javan gibbon is found only in forest remnants of western (H. moloch
moloch/H.moloch pongoalsoni) and central Java (H. moloch pongoalsoni) in the remaining
lowland and lower montane forest (Andayani, Morales, Forstner, Supriatna, & Melnick,
2001; Supriatna et al., 2010). It is estimated that the Javan gibbon has lost up to 96 percent
of its original habitat and should receive the highest priority of protection for Asian primates
(Malone, 2007; Supriatna, 2006).
Javan gibbons tend to exhibit a socially monogamous mating pattern (i.e., the adults
may not be sexually exclusive) (Marshall, 2009). They generally live in small, familial
social units consisting of an adult pair and their dependent offspring, typically averaging
three to five individuals (Bartlett, 2007; Fuentes, 2000). The onset of sexual maturity for
females is on average eight and half years, and around age ten for males. The age at first
birth for females usually occurs around age nine or ten (Brockelman, Reichard, Treesucon, &
Raemaekers, 1998). Interbirth interval falls between two to three years (Hodgkiss, Thetford,
Waitt, & Nijman, 2009). Adult gibbons were traditionally believed to force their same-sexed
offspring out of the family group upon sexual maturity via aggressive behavior (Chivers,
1978; Fuentes, 2000). However, the dynamics of dispersal (e.g., the age and sex) by young
adult gibbons has been shown to vary and they may disperse as near as the neighboring
group, leading to potentially a certain degree of relatedness amongst neighboring family
groups (Brockelman et al., 1998; Fuentes, 1999, 2000). Furthermore, there is evidence that
young adult gibbons may be tolerated within their groups for at least two years (possibly
more) past sexual maturity and they may benefit the group by aiding in territorial defense
7
(Brockelman et al., 1998). There is also increasing evidence of behavioral flexibility and
higher levels of affiliative interactions between neighboring groups across the Hylobatidae,
including Javan gibbons, and intergroup encounters are not always agonistic (see Bartlett,
2007; Malone, 2007). Bartlett (2003) suggests that intergroup encounters between adult
gibbons provide an opportunity for the young adult gibbons to meet and identify potential
mates and therefore, facilitating dispersal from their natal group. Data collected from field
studies has shown that there is a significant rate of movement by individual gibbons across
groups and home ranges, suggesting a larger, more fluid, social network among groups of
gibbons (Malone, 2007). Though gibbons generally do spend the majority of their time
living in a two-adult unit, there have been observations of gibbons living solitary, in greater-
than-two-adult groups, and movement between those groups (Fuentes, 2000; Palombit,
1994). In addition, gibbons are known to engage in extra-pair copulations with individuals
from adjacent territories (Reichard, 1995). Malone (2007) observed at least one group of
wild Javan gibbons living in a multi-male/uni-female group in Cagar Alam Leuweung
Sancang (CALS) in West Java, Indonesia.
Javan gibbons are one of only two gibbon species not known to produce duet songs
(Kloss’s gibbon (Hylobates klossii) being the other). Instead, most of the singing is produced
by females and mated females appear to be the vocal “representative” of the family
(Geissmann, 2002). Male Javan gibbons will sometimes produce their own specific
vocalizations, or contribute to the female’s great call, especially during territorial disputes
(Malone, 2007). Although gibbons are considered to be territorial, they do not necessarily
defend their entire home range. Both male and female Javan gibbons will attempt to defend
their territories by engaging in vocal and/or physical displays and chasing intruders out.
Each family group typically occupies a home range of approximately 15- 37 hectares (Kim,
Lappan, & Choe, 2010; Malone, 2007; Nijman, 2004, 2006; Supriatna, 2006). The home
range of Javan gibbons may vary depending on resource abundance, habitat disturbance, and
altitude. Furthermore, these factors also influence the daily path length (dpl) exhibited by
Javan gibbons, which ranges from 835 – 1,400 meters/day (Kappeler, 1981; Kim et al., 2010;
Malone, 2007). Javan gibbons are exclusively arboreal and prefer the upper canopy of
lowland ( 500 m asl) to lower montane tropical forest (1,000 – 1, 500 m asl) (Kim et al.,
2010; Malone, 2007). Lowland forests tend to exhibit higher plant diversity and fruit
8
availability, as well as a higher density of large trees having a diameter at breast height (dbh)
of greater than or equal to ten centimeters (Kim et al., 2010). The majority of Javan gibbons
in the wild inhabit hill (500-1,000 m asl) and lower montane forest (Kim et al., 2010).
Gibbons are the most suspensory of all the primates and utilize brachiation as their
main form of locomotion. However, they will also traverse through the canopy by climbing,
leaping, and running or walking bipedally along the branches. Brachiation is a unique form
of movement in which the gibbon uses its pectoral limbs to support the full weight of its
body in suspension beneath a superstratem, as it moves utilizing a hand over hand
locomotion along a branch. They have precise adaptations for maximizing the forward
momentum gained from the pendular motion of the body during brachiation (Cheyne, 2004;
Fleagle, 1976). The gibbons’ thumb is greatly reduced to hook onto branches during
brachiation, rather than grasping, allowing them to move at a greater pace through the
canopy. In addition, they have a well developed scapular spine, long forearms relative to
their body size, and they also raise and lower their legs in order to maximize momentum
while brachiating (Fleagle, 1976). In order for gibbons to efficiently travel through the forest
they require primary, continuous canopy forest with dense foliage and horizontal growth
(Asquith, 1995; Bartlett, 2007). Only captive-raised gibbons have a tendency to travel on the
ground (Cheyne, 2004). Gibbons and siamangs are considered the only true brachiators of all
the nonhuman primates, moving through the canopy at speeds as high as 40 kilometers per
hour, leaping as far as 15 meters, and traveling as far as 6 meters with each swing (Fleagle,
1988). They are incredibly agile and exceptional acrobats when traversing through the
forest.
Gibbons are predominantly frugivorous, but will also consume insects, flowers, and
leaves (Bartlett, 2007; Groves, 1984; Leighton, 1987). They require a wide variety of tree
species that will fruit at different times of the year to support their dietary needs. They are
one of the most important frugivores in Southeast Asian forests, typically dispersing over 81
percent of the species they consume (Bartlett, 2007; McConkey, 2000). They tend to
disperse few seeds under parent trees with more than 90 percent distributed over 100 meters
away (McConkey, 2000). McConkey (2000) also reported that gibbons actually improve the
chance of germination of some of the species in their diet; many fig seeds will not germinate
without prior passage through the gut. It is very unlikely that gibbons will inhabit secondary
9
forest or cross open sections of forest, thus they will contribute very little to forest
regeneration in secondary forests by seed dispersal. Secondary forests have gaps in the
canopy and the growth is sparse, thus restricting the gibbon's ability to efficiently move
around. There is also less of a variety of fruiting trees in new growth forest (McConkey,
2000) which potentially cannot support the dietary needs of the Javan gibbon. Therefore, the
role of gibbons, and primates in general, in maintaining forest biodiversity is an area of
increasing interest and has significant implications for conservation programs, including
reintroduction efforts and increasing the amount of protected areas in primary forest (Bartlett,
2007; Chapman, 1995). Educating local communities about the importance of primates in
maintaining forest biodiversity via seed dispersal, may aid in gathering more support for
reintroduction programs and habitat protection.
In comparison to other small bodied primates, gibbons have very few natural
predators (i.e., leopards, birds of prey, pythons) and a relatively low predation rate, with the
exception of humans (Kappeler, 1981). As a result, gibbons are able to avoid predation
mostly by remaining in the upper levels of the forest canopy. Gibbons are rarely found less
than ten meters below the ground. Leopards and pythons are most often found in the lower
levels of the forest canopy (Kappeler, 1981). Most species of gibbons, particularly Javan
gibbons with their silvery gray fur, are well camouflaged within the canopy. Gibbons are
naturally vigilant creatures, exceptionally so during feeding and grooming where they will
actively scan the area in an attempt to avoid possible predators (Kappeler, 1981). After
gibbons engage in singing or other noisy behavior, they tend to quietly move on to a new
area of the forest in an attempt to avoid being discovered by potential predators (Kappeler,
1981).
THREATS TO THE SURVIVAL OF JAVAN GIBBONS
The following section provides an overview of the threats to the survival of Javan
gibbons in the wild.
Loss of Habitat/Habitat Fragmentation
Indonesia belongs to the Sundaland Biodiversity hotspot (Mittermeier et al., 2005;
Supriatna et al., 2010). It is rich in endemic species and has the third largest forested area in
the world. Unfortunately, Indonesia is also experiencing the highest rate of deforestation in
10
the world and its ecosystems are constantly under threat from human activities (Mittermeier
et al., 2005; Siscawati, 2010). The deforestation rate in Indonesia is almost three times
higher than the average rate of tropical deforestation in the world. There is only about 37
percent of the original primary forest remaining out of the total forest area that existed in
1966. Prior to 1966, Indonesia had not yet suffered from structural adjustment programs,
debts, and aggressive private capital flows (Siscawati, 2010). These contributed to policies
including inequitable land tenure and incentives that lead to deforestation and forest
degradation. The accompanying social, economic, cultural and spiritual exploitation caused
the suffering of more than 40 million indigenous people and many local communities in
Indonesia (Siscawati, 2010).
The island of Java is one of the most densely populated areas in the world with over
115 million inhabitants and an average population density of more than 900
people/kilometers² (Nijman, 2006; Supriatna, 2006; Supriatna et al., 2010). Java has had a
long history of cultivation and deforestation that began in 1000 AD. It is now largely
deforested and the majority of the forest fragments cover parts of the many volcanoes on the
island. The remaining land of Java is a mosaic of cities, villages, and rice fields (Nijman,
2006). The Dutch arrived on Java in 1830 and began heavily exploiting the natural resources
of the island. The Dutch colonial government initiated a state forestry system and required
farmers to grow export crops on communal grounds, which was often the forest (Nijman,
2004; Peluso, 1993). By the end of the 19th century the natural forest was severely
fragmented, especially in western and central Java. Presently, the major cause of loss to
remaining forest is due to illegal small-scale logging (i.e., tree cutting for subsistence plots,
firewood, forest fires, and charcoal production), much of what goes unpunished by law, as
opposed to the industrial scale logging of the past (Malone, 2007; Smiet, 1992; Supriatna,
2006; Supriatna et al., 2010). It is estimated that less than ten percent of the original forest
on Java remains, including 19 percent of hill forest (500-1000 m asl), and only two percent of
the lower montane forest (<500 m asl) is still present (Nijman, 2004). The Javan gibbon
prefers relatively undisturbed lowland and lower montane rainforest, ideally below 1600
meters. Due to human encroachment, gibbons are being pushed up into higher elevations
where the habitat is less suitable for them (Nijman, 2004). However, there are groups in
Cagar Alam Leuweung Sancang (CALS) that inhabit edge and disturbed forest fragments
11
(Malone, 2007). The behavioral and ecological flexibility demonstrated by some Javan
gibbons, may pave the way for future studies with regards to their response to anthropogenic
disturbance.
The Indonesian government owns most of Java's forests and manages them through
the State Forestry Corporation (Peluso, 1993). Teak wood is still exported by Java, but is
regulated by the government which sets rules about the size and number of trees to be
exported. Environmental groups have applauded the management of teak plantations, largely
due to the improvement of well managed rotation of the crops and the government's attempt
to implement a social forestry program. The social forestry program was created with the
goal of including local people in forest resource management, but has not been entirely
successful (Hidayat, 2008). In general, local people are only given a limited amount of
control and only if they participate in the system working as forest laborers (Hidayat, 2008;
Peluso, 1993). In 2001, legislation for regional autonomy went into effect and forest
management was given over to local governments, except for in conservation areas
(Supriatna et al., 2010). Local governments tend to focus on short-term economic gains such
as logging, instead of long-term sustainable management of natural resources (Malone, 2007;
Supriatna, 2006; Supriatna et al., 2010). The decentralization of the government has both
positive and negative implications for conservation and the livelihood of the local people.
The positive outcome is the increased ability for NGOs to lobby for local regulations that
recognize the indigenous rights to natural resources and promote sustainable forest use. On
the negative side, local districts can now issue a larger number of permits for local companies
that may potentially exploit the forests (Supriatna, 2006; Supriatna et al., 2010). The local
people, including the decision makers, are not provided with adequate information regarding
the importance of conservation and the long term benefits they can derive from the forests
(i.e., watershed protection) (Supriatna, 2006). Moreover, the concept of forest management
developed by the Department of Forestry itself does not ensure conservation. The logging
companies never comply with their own policies in consistently implementing forest
management concepts (Hidayat, 2008). Concessionaires breach the limits of logging areas
and illegal logging of small diameter trees is permitted. In addition, they do not replant or
renew trees and they forge timber transport documents, as well as log on upland areas.
12
Therefore, the management concept of Indonesian forests has not been entirely successful
and the forests continue to disappear (Hidayat, 2008).
It is imperative to raise awareness for the local people in regards to the benefits they
will appreciate from sustainably utilizing forest resources. The rich forests of Gunung Gede-
Pangrango, Gunung Halimun, and Gunung Salak are primary water catchment areas for over
20 million people in the surrounding area, including the city of Jakarta. It is crucial to
protect these forests in order to ensure a long time water supply for the people (Supriatna et
al., 2010). Furthermore, Javan gibbons, like all of the hylobatids, tend to prefer full, closed
canopy within mature forest for sleeping, eating, and singing. Female gibbons prefer taller
trees that extend beyond the canopy for performing their 'great calls' and accompanying
displays (Whitten, 1982). Gibbons can be sensitive to disturbance and will often alter their
behavior in response to frequent logging, collection of forest resources, human
encroachment, or hunting (Nijman, 2006). This behavior can potentially be manifested in
several ways, including an increase in vigilant behavior resulting in avoidance, retreat or
hiding behaviors, the gibbons may move lower in the canopy, and ultimately, suppress their
vocalizations (Malone, 2007; Nijman, 2006). Gibbons produce loud vocalizations in order to
defend their territory, attract mates, and potentially reinforce the pair bond. Although Javan
gibbons are not known to duet, males will occasionally contribute to the females’ great call
or during territorial disputes (personal observation) and this could be considered a form of
pair bond reinforcement. In addition, calling frequency in gibbons is dependent on, amongst
others, the density of gibbons (Chivers, 1978). If wild gibbons are suppressing their
vocalizations, or engaging in other types of cryptic behavior due to anthropogenic
disturbance, it may prove difficult for researchers to obtain an accurate count of gibbons in
the wild, which can influence conservation strategies aimed at protecting them.
In 2008 the IUCN downlisted the Javan gibbon from Critically Endangered to
Endangered. This recommendation came from more intensive surveys that were conducted
in the western and central part of the island that indicated a larger population in the wild than
was previously believed (Nijman, 2004). The underestimation of Javan gibbons in the wild
seemingly was the result of methodological differences in population estimation (i.e., actual
numbers of gibbons observed in a few areas vs. extrapolation from density estimation at
varying geographic and altitudinal zones) (Malone, 2007; Nijman, 2004). Based on a review
13
of the literature, there has been a variety of methods used to determine the actual number of
Javan gibbons left in the wild, all yielding different estimates. Methods range from rapid
presence-absence surveys, range mapping, fixed line transects to fixed-point counts (Nijman,
2004) and population estimates have ranged from as few as 400 individuals to as many as
7,900 individuals remaining in the central and western parts of Java (Kappeler, 1981;
Nijman, 2004). The current population is estimated at 4,000-4,500 Javan gibbons (Nijman,
2004; Supriatna et al., 2010). Wild Javan gibbons prefer to avoid humans when they
encounter them in the forest. This could potentially make it more difficult for researchers to
actually detect them in the wild and may lead to inaccurate counts of individuals (Nijman,
2004). Most primates in the wild are elusive, however, gibbons live in smaller groups and
therefore, tend to be more difficult to detect in the wild. If the number of gibbons are
estimated too low in the wild, this could potentially lead to focusing conservation efforts
more on saving the ex-situ population (i.e., the captive population), as opposed to providing
more protection in their natural habitats (in-situ) (Nijman, 2004). It could potentially be
beneficial to focus efforts on establishing a captive breeding population /metapopulation that
could then be used to supplement the population of Javan gibbons in the wild.
Illegal Pet Trade
It is believed that the equivalent of an entire population of Javan gibbons
(approximately 300 individuals) is held illegally in captivity in Indonesia (Nijman, 2006;
Supriatna, 2006). The north coast of the island of Java is a major route for the trafficking of
Indonesian nonhuman primates, possibly including the Javan gibbon (Malone, Fuentes,
Purnama, & Adi Putra, 2004; Supriatna, 2006). The majority of Javan gibbons held in
captivity in Indonesia are directly derived from the wild, as opposed to captive breeding
programs, including the individuals who are in zoos and wildlife rescue centers (Nijman,
2006). When infant gibbons are removed from the wild, it is usually after their mother (and
perhaps other family members) have been shot and killed. This has potentially detrimental
effects on the wild Javan gibbon population because aside from the young gibbons being
removed, viable female gibbons are also being removed from the total population (Malone,
2007). Furthermore, if the infant does survive the pet trade and become a household pet, it
will most likely never have the opportunity to reproduce and contribute to any population of
14
wild Javan gibbons. One of the major challenges in the enforcement of illegal wildlife trade
regulations is the willingness of the authorities to become engaged in and carry through the
required judicial procedures (Supriatna et al., 2010). As a result, current offenders of wildlife
laws are rarely prosecuted (Malone, 2007; Nijman, 2006; Supriatna, 2006). The current
system in Indonesia allows for individuals to donate their pet gibbons to zoos, wildlife rescue
centers, and rehabilitation programs when they realize they can no longer manage the gibbon
as a pet. As a result, it becomes easier for private owners to dispose of their adult gibbon and
potentially obtain a younger individual from the wild, perpetuating the illegal pet trade
system (Supriatna, 2006). Malone (2007) provides an excerpt from a report discussing a
potential link between the Ministry of Forestry and the illegal trade of wildlife, including
Javan gibbons:
Contesting the official lament about the plundering of Indonesia's natural wealth is Iwan Setiawan, staff member of the Indonesian Environment Information Center (PILI). In September, he led a team to research the trafficking in endangered species in the Pramuka and Barito pet markets. He said he found some protected animals on sale and evidence of transactions, and reported his findings to the ministries of Forestry and Environment, the Nature Conservation Body and the police. The result? Nothing, said Iwan, which has led him to conclude that there is a nexus between the smugglers of endangered species and the authorities. It is a view that is repeated by the Gibbon Foundation, a conservation group based in Jakarta. A member of the group's office explained that the foundation had at one time entered into a cooperation agreement with the Forestry Ministry to map wild-animal populations using satellite imagery. The aim was to identify the remaining concentrations of wild species. “But this invaluable information was leaked out,” said the staffer, “then we found traces of professional hunters in the regions and the animals had gone.” (Malone, 2007, p. 32)
The illegal pet trade is unfortunately a valuable mode of profit for individuals in
Indonesia. Internal exploitation and corruption is undoubtedly contributing to deforestation
rates in the country, as well as the disappearance of wildlife, and is the result of a system that
perceives natural resources as a means of revenue that can be exploited for political and
social gain (Malone, 2007). The ownership of an endangered nonhuman primate is portrayed
as a symbol of status and economic success. It is reported that many government officials,
army officers, and entertainers own pet nonhuman primates, especially orangutans (Malone
et al., 2004). This practice unfortunately encourages other people to purchase nonhuman
primates, and enhances the symbolic status of owning an endangered species. As a
15
consequence, the reported public display of nonhuman primate pets by the wealthy and
influential serves as a demonstration of political power, and ultimately, the selective
enforcement of Indonesian law (Malone et al., 2004). This information exemplifies the
challenges that conservationists face when developing conservation programs that seek to
preserve the remaining forests and wildlife of Indonesia. Furthermore, regarding the primary
issues of habitat loss and the illegal trade in gibbons, local people will have to be involved in
the long-term effective management of forest resources and they must demonstrate the will to
report and prosecute individuals who are involved in the illegal pet trade (Malone, 2007).
Small Population Processes
Due to the severe fragmentation of Java's forests, the only suitable habitat remaining
on the island for Javan gibbons is small isolated patches of forest. The fragmented nature of
this remaining habitat makes the species highly vulnerable to extinction via small population
processes, including environmental stochasticity, demographic stochasticity, and loss of
genetic diversity (Cowlishaw & Dunbar, 2000). In general, extinction risk is a function of
population size. If populations are large, the risk of extinction is relatively low, but if species
exist in smaller populations, the risk of extinction becomes much greater (Cowlishaw &
Dunbar, 2000). If gibbons are unable to disperse into new territory and acquire new mates,
the isolated populations could potentially be at great risk for extinction through loss of
genetic variability. A reduction in genetic diversity of a population (i.e., decreased
heterozygosity, increased homozygosity) is considered undesirable for two reasons. The first
reason being, the loss of heterozygosity means that populations have less genetic flexibility
allowing them to respond to changes in the environment. Secondly, there is a potential
increase in the likelihood of inbreeding depression (i.e., the phenotypic expression of
deleterious recessive genes). As population size decreases, the effect of such an impact
becomes disproportionately severe (Cowlishaw & Dunbar, 2000).
In order to increase genetic diversity in small populations of Javan gibbons, it could
prove beneficial for conservation efforts to translocate gibbons between forest patches and
establish a metapopulation between wild and captive individuals. Researchers have
suggested the need of establishing a metapopulation management program through some
form of genetic supplementation. This would include capturing young individuals who are
16
preparing to leave their natal groups and transferring them to other areas (translocation)
(Nijman, 2004, 2006). This conservation strategy was utilized in an effort to save the golden
lion tamarin (Leontopithecus rosalia) from going extinct in the Atlantic coastal forest of
Brazil. Similar to Javan gibbons, golden lion tamarins are found only in isolated forest
patches and are vulnerable to extinction via small population processes. The golden lion
tamarin wild population was estimated at only 100-200 individuals in the 1970s. A
metapopulation was established and the first captive-bred tamarins were released in the mid
1980s, and by 1994 the wild population had grown to approximately 450 individuals with an
additional 550 in captivity (Ballou et al., 2002; Britt, Welch, & Katz, 2003; Kleiman, 1989).
For Javan gibbons, another conservation strategy that may prove beneficial in supplementing
the wild populations is the rehabilitation and reintroduction of individuals from rescue
centers into isolated forest areas without resident gibbons (Nijman, 2006; Supriatna, 2006).
The reintroduction of Javan gibbons back into their native habitat, as well as the
establishment of a metapopulation, may prove to be a more complicated conservation
strategy based on the fact that there are potentially two identified subspecies living on the
island. In a molecular genetics study, Andayani et al. (2001), found that two lineages of
Javan gibbons may exist: a western lineage (H. moloch moloch) that is represented by a large
population in Gunung Halimun and an central lineage (H. moloch pongoalsoni) consisting of
isolated populations around the Gunung Masigit/Simpang/Tilu complex, Gunung Gede-
Pangrango, and Gunung Slamet in central Java (Andayani et al., 2001; Asquith, 1995;
Supriatna, 2006) (see Figure 1). The morphological differences between the two populations
are quite subtle, so additional research on the phylogenetic analyses of mtDNA and
vocalizations may shed further light on the finding (Mootnick, 2006; Supriatna et al., 2010).
Furthermore, to the best of my knowledge, there are not published scientific data from wild
studies as to exactly how the two potential subspecies of Javan gibbons differ in terms of
their behavior and ecology. According to the study conducted by Andayani et al. (2001), the
discovery of the two lineages has potential implications for conservation policy and
reintroduction programs and they make the following recommendations: (1) the western
population should be managed as a separate and distinct population unit and should not be
used to reinforce the central population; (2) the gibbons from the Gunung
Masigit/Simpang/Tilu complex, Gunung Gede-Pangrango, and Gunung Slamet represent a
17
Figure 1. Map illustrating the distribution of the western and central population of Javan gibbons according to Andayani, N., Morales, J. C., Forstner, M. R. J., Supriatna, J., & Melnick, D. J. (2001). Genetic variability in mtDNA of the silvery gibbon: Implications for the conservation of a critically endangered species. Conservation Biology, 15, 770-775. Darkened areas represent forest areas where gibbons are still present. Outlined areas represent potential suitable habitat, but are uninhabited by wild gibbons. Kappeler (Kappeler, M. (1981). The Javan silvery gibbon (Hylobates lar moloch). (Doctoral dissertation) Universitat Basel, Basel, Switzerland.) surveyed Gunung Salak and found no gibbons (adapted from Andayani, N., Morales, J. C., Forstner, M. R. J., Supriatna, J., & Melnick, D. J. (2001). Genetic variability in mtDNA of the silvery gibbon: Implications for the conservation of a critically endangered species. Conservation Biology, 15, 770-775).
second distinct unit and can be moved between the surrounding areas; and (3) the gibbons
from Gunung Slamet are not evolutionarily distinct from the gibbons directly to the west.
However, the gibbons from Gunung Slamet do merit special attention because they may
represent a case of peripheral isolation (Andayani et al., 2001; Supriatna, 2006). Stanford
(2001) argues that splitting primate taxa into subspecies should be done cautiously and
conservatively, because it will heavily influence conservation initiatives. There should be a
18
consensus of biogeographic, genetic, morphological, behavioral, and ecological factors that
are used to distinguish the subspecies from one another, all of which are limited (if not
absent) for Javan gibbons. Considering Andayani et al. (2001) represents the only molecular
study conducted on Javan gibbons to date, it would seem necessary to engage in further
behavioral and ecological studies on determining the level of distinction between the two
subspecies, prior to developing conservation policies aimed at protecting them in the wild.
Groves (2001), Mootnick (2006), and Geissmann, Dallmann, and Pastorini (2002) do not
acknowledge the subspecies level of classification for the Javan gibbon. Also, the 2006
Asian Primate Red List workshop assessed the conservation status of all species of gibbons
and the Javan gibbon was listed as Hylobates moloch, with no regards to the two lineages.
This assessment would seem to suggest that conservation initiatives aimed at preserving them
in the wild are considering the Javan gibbon to be a monotypic species. Supriatna et al.
(2010) suggests that reintroduction efforts should be conducted with caution until further
genetic and vocalization studies have verified the evidence of the two lineages of Javan
gibbons and if there are any detrimental effects if the two subspecies were to interbreed.
This recommendation potentially raises the issue of the validity of the subspecies
classification regarding conservation measures, particularly for species that are considered
highly endangered (Smith, Chiszar, & Montanucci, 1997). Considering the threats to Javan
gibbons in the wild are persistent, should conservation efforts focus on preserving the Javan
gibbon at the species level, or attempt to preserve the two lineages? I would argue that
conservation efforts should prioritize eliminating the extrinsic threats, loss of habitat and the
illegal pet trade, before we can afford the luxury of managing the two subspecies separately.
It is important to consider the feasibility of the subspecies concept with regards to
reintroduction efforts. It would be necessary for JGC to determine the lineage to which each
Javan gibbon belongs prior to release, and then this would determine where the pair was to
be reintroduced in the wild. However, at this time, JGC does not identify the subspecies
level of classification for the individuals at JGC prior to placing them in pairs or before
locating a potential release site. Considering there are perhaps as few as 4,000 to 4,500
Javan gibbons remaining in the wild, if we could obtain an accurate population count for
each potential subspecies, perhaps a higher level of conservation priority aimed at protecting
the species and its habitat in the wild could be developed.
19
Riley (2010) presents information on a similar scenario regarding the conservation
status of the crested black macaque (Macaca nigra) on the island of Sulawesi. Currently, it
is estimated that only 4,000 to 6,000 crested black macaques remain on Sulawesi. However,
there is a larger, introduced population of approximately 100,000 individuals on Bacan Island
(~300 km east). Although the population of crested black macaques on Sulawesi and the
Bacan population are essentially identical with regards to their behavior and morphology,
genetic analysis (i.e., dermatoglyphics, polygenetic differences) has revealed a significant
divergence from the mainland population (Froehlich, Supriatna, & Muskita, 1996). In spite
of this finding, the crested black macaque is considered a monotypic species. However,
Froehlich et al. (1996) state the Bacan macaques should not be considered a genetic reservoir
for the threatened mainland populations, unless they become totally extinct on Sulawesi. As
of present, the IUCN does not include the Bacan population in its conservation assessment of
the crested black macaque, in order to maintain the focus of conservation priority on
eliminating the extrinsic threats to the survival of the species on Sulawesi (Riley, 2010).
Rehabilitation and Reintroduction
The American National Wildlife Rehabilitation Association defines rehabilitation as
“the treatment and temporary care of injured, diseased and displaced indigenous wildlife, and
the subsequent return of healthy viable animals to appropriate habitats in the wild”
(Atkinson, 1997; Cheyne, 2004). The IUCN (2002) defines reintroduction as the
“introduction of a species in a previously occupied area in order to improve the conservation
of the species” (p. 6). It is crucial that the individuals that are to be released have either
maintained or developed the appropriate behavioral repertoire necessary for survival in the
wild, and have gone through an extensive period of rehabilitation (Cheyne et al., 2008).
Reintroduced individuals may be translocated from other wild populations or may come from
captive breeding programs (IUCN, 2010; Sarrazin & Barbault, 1996). In its
recommendation section for reintroduction, the IUCN (2002) states that programs should
incorporate feasibility studies and preparatory, and pre-and post-release monitoring periods.
In order for a reintroduction program to be successfully implemented, there must be adequate
funding, local agreement, and the population viability of a species (i.e., demographic,
genetic, behavioral, and ecological processes) within their natural environment must be
20
thoroughly understood (Sarrazin & Barbault, 1996). A population viability analysis typically
includes an examination of a species’ characteristics and environmental variability in order to
determine population health and extinction risk (IUCN, 2010). The reintroduction of
endangered species to their original habitat is often a very attractive, although expensive and
highly involved, measure for conservation and is sometimes used as a last resort for saving
species that are almost extinct in the wild (Ballou et al., 2002; Kleiman, 1989).
Unfortunately, through the years very few reintroduction programs have been successful,
presumably due to inadequate pre-release preparation and post-release monitoring (Cheyne,
2004; Stoinski, Beck, Bloomsmith, & Maple, 2003). In order to gain insight on how species
adapt to the rehabilitation process, and to ensure that they have acquired the appropriate
behavioral repertoire and survival skills that can then be compared when they are released, it
is imperative that individuals are monitored during both the pre-and post-release phases of
the project (Cheyne et al., 2008).
Reintroduction programs involving nonhuman primates have been in place since the
early 1960s. In the 1970s perhaps one of the most successful captive breeding/reintroduction
programs was established with the golden lion tamarin in Brazil’s Atlantic coastal forest
(Ballou et al., 2002; Britt et al., 2003). The golden lion tamarin became the first example of
a “flagship species” that served to ignite conservation efforts not only for the species itself,
but also for the Atlantic coastal forest ecosystem as a whole. The golden lion tamarin wild
population was estimated at only 100-200 individuals in the 1970s due to extensive habitat
destruction and the illegal pet trade (Ballou et al., 2002). The first captive-bred tamarins
were released in the mid 1980s, and by 1994 the wild population had grown to approximately
450 individuals with an additional 550 in captivity (Ballou et al., 2002; Britt et al., 2003;
Kleiman, 1989).
A common challenge in rehabilitating nonhuman primates is they have often had very
little exposure to their native habitats before they were subjected to a life in captivity (usually
via illegal pet trade), and therefore lack the appropriate knowledge and experience necessary
for surviving in the wild (Britt et al., 2003). In field studies, zoo observations, and laboratory
experiments the great apes (as well as monkeys) have demonstrated a great capacity to learn
in a complex and novel environment. The reintroduction programs with the great apes,
however, specifically orangutans, have been somewhat of a challenge because orangutans
21
tend to imprint very heavily on their human caretakers. Considering they tend to have
extensive relationships with their mothers lasting seven to eight years in the wild, this is often
difficult to negotiate when releasing orphaned orangutans, even after a period of
rehabilitation, because they have been deprived of this vital relationship with their mothers
(Grundman, 2006). Orphaned orangutans, and chimpanzees, will often return to the rescue
center in search of food, not show any fear of humans, and do not always engage in the
appropriate social behavior with conspecifics. Released orangutans have been observed
remaining in small groups, approaching wild orangutans and entering their sleeping nests at
night, and even attacking humans (Grundman, 2006; Yeager, 1997). This is problematic for
the reintroduction program because these individuals are not adapting to a life free of human
dependency and will most likely not contribute to the preservation of the wild population(s)
of orangutans in anyway (i.e., not successfully reproduce in the wild).
In June 2009, the first ever group of rehabilitated bonobos (Pan paniscus) from Lola
Ya Bonobo were released into a 20,000 hectares area of swampy forest in Equateur Province,
Democratic Republic of Congo (Les Amis des Bonobos du Congo Newsletter, 2009). There
were nine individuals released, including one pregnant female. Upon release the group
remained cohesive and began to explore their new forest environment. The bonobos were
continuously monitored by staff from the local community and minimally provisioned with
food and water. The bonobos were observed making more sophisticated nests than they
previously had in captivity, they were successful at locating and procuring food sources, and
engaged in appropriate social behaviors with one another. However, some of the individuals,
specifically one of the adult males, were displaying aggressive behavior (approaches,
physical displays) towards humans. After an intruding villager was bitten by one of the
bonobos, the three individuals who were showing the most aggressive behavior towards
humans were transported back to Lola Ya Bonobo. The remaining individuals still frequent
the release location for nesting and feeding, thus providing the researchers with an
opportunity to directly observe the bonobos. The bonobos can easily be observed from the
nearby river allowing the local villagers a chance to observe them from a closer distance.
This has helped increase public awareness regarding conservation efforts for the bonobos,
and has had a very positive effect on the local communities. None of the villagers had
actually ever seen a live bonobo before, and willingly admitted that if they had ever eaten
22
bonobo meat before, they vowed never to do so again after realizing their close resemblance
to humans (Les Amis des Bonobos du Congo Newsletter, 2009).
Compared to the great apes there has been very little attention given to reintroduction
programs for gibbons. However, the issues of what is involved in successfully rehabilitating
a gibbon remain the same as for the great apes. Gibbons who have been kept as pets develop
a very different behavioral repertoire than that of wild gibbons (Mootnick & Nadler, 1997).
Different skills are needed for survival in the wild, and gibbons would normally acquire these
valuable skills, such as how to locate and identify food, avoid predators, navigate in a
complex environment, locate sleeping/singing trees, and socialize with conspecifics from
their parents. Wild gibbons may remain in their natal group for up to twelve years,
sometimes longer, so when infants are removed from the wild, it can be extremely
detrimental to their social and psychological well being (Mootnick & Nadler, 1997). Former
pet gibbons (i.e., human raised) have been deprived of this crucial learning period and must
depend on humans to assist them in acquiring these skills (i.e., placing individuals into
appropriate social environments, incorporating wild foods into the gibbons’ diet, and
providing an enclosure that will help facilitate proper forms of locomotion) (Cheyne et al.,
2008; Grundman, 2006). Another potential issue with former pet gibbons is the prevalence
of stereotypic or human-directed behaviors that arise from a life in captivity within an
unnatural environment with human contact/interaction. These behaviors should be severely
minimalized or eradicated before the gibbons are released into the wild. If rehabilitated
gibbons solicit attention from humans, whether in an affiliative or an aggressive manner, it
could potentially be harmful to them, or humans, and hinder their chance for survival. It is
imperative that we have a better understanding of the gibbons’ behavior throughout the entire
rehabilitation process, so that we can help facilitate the reintroduction phase of the project
and ensure the gibbons are able to survive without human intervention once they are released
into the wild (Cheyne, 2004).
Some of the early attempts to reintroduce gibbons were unsuccessful primarily due to
poor planning and management practices. For example, in Sarawak, Malaysia on the island
of Borneo, the reintroduction effort of Mueller’s gibbons suffered a 90 percent mortality rate
in released individuals. Several factors may have been the cause of failure: hunting in the
area, starvation, disease, and conspecific aggression (Cowlishaw & Dunbar, 2000). The
23
release of lar gibbons as reported by Tingpalapong, Watson, Whitmire, Chapple, and
Marshall (1981) saw far better rates of success. Thirty-one former laboratory gibbons were
released into closed forest in Thailand. The released individuals consisted of 26 wild caught
individuals and five offspring that were captive born. This reintroduction program was more
cautious in the pre-release planning by selecting forest that already had resident gibbons,
although not many so there were vacant territories for the released gibbons, and the area was
protected from hunting. Researchers monitored the gibbons post-release and provided the
gibbons with food and shelter. The gibbons were released in family groups, pairs, or
individuals. Unfortunately, two of the gibbons died at the release site, one was recaptured,
24 disappeared over time, and four of the gibbons joined wild groups. It is assumed that the
most successful individuals were those in the first group to be released, because they were
able to establish their own territories within the present population of gibbons (Cowlishaw &
Dunbar, 2000).
GIBBON RESCUE AND REHABILITATION CENTERS
As of present, there are only three rescue and rehabilitation centers in Asia devoted
strictly to gibbons; the Gibbon Rehabilitation Project in Phuket, Thailand; Kalaweit gibbon
center in Kalimantan, Indonesia; and the Javan Gibbon Rescue and Rehabilitation Center in
West Java, Indonesia.
GIBBON REHABILITATION PROJECT (GRP)
The Gibbon Rehabilitation Project (GRP) is run by the Wild Animal Rescue
Foundation of Thailand and was established in 1992 by Mr. Noppadol Preuksawan, the chief
of the Royal Forest Department in Phuket at that time, Mr. Thavrn SriOon, Bang Pae Sub-
Station chief, the Asian Wildlife Fund and an American zoologist, Terrance Dillon Morin
(The Wild Animal Rescue Foundation of Thailand, 2010). The objectives of the
reintroduction program are to: develop a method to rehabilitate lar gibbons back into their
natural habitat; end the demand for the illegal use of gibbons as tourist attractions and as
pets; create awareness to the importance of conservation of the environment; and provide the
opportunity for volunteers to study the gibbons. According to the website, the reintroduction
program has been quite successful in terms of overall survivability of the released gibbons.
The center does not provide any scientific data to support this claim, only brief summaries on
24
the website and the center does not conduct long term post-release monitoring of the gibbons.
Cheyne (2004) states that the center is in a serious need of a complete overhaul of its
infrastructure and its approach to the rehabilitation/ reintroduction process. The GRP
advertises that it is seeking volunteers to assist in the reintroduction program, but has never
provided the scientific community the opportunity to monitor the released gibbons (Cheyne,
2004). Critics of the GRP (Cheyne, 2004; de Veer & van den Bos, 2000; Schoene & Brend,
2002) state that the center has created more welfare problems for the gibbons by exceeding
its carrying capacity and engaging in poor management practices. The rehabilitated gibbons
were not always released in established pairs. The island itself was of poor habitat quality
(not contiguous forest) and not capable of supporting the gibbons long term, resulting in long
term provisioning. Ultimately, the gibbons are dependent on humans, living in poor habitat,
and not contributing to the wild population (Cheyne, 2004).
KALAWEIT GIBBON CENTER (KP)
The Kalaweit gibbon center (KP) was established in 1999 and has centers on both
Borneo (Kalimantan) and Sumatra. It is currently the largest rescue and rehabilitation center
for gibbons (white-bearded and Mueller’s gibbons) and siamangs (Symphalangus
syndactylus) in the world, housing approximately 300 gibbons and siamangs between the two
locations (Kalaweit Gibbon Center [KP], 2010). Although the center is not open to the
public, volunteers are permitted to go to the center and work as primate keepers and help
with the reintroduction efforts for the gibbons. Kalaweit includes local people in all aspects
of the reintroduction program by only hiring people from the surrounding villages to work at
the center. In addition to the reintroduction program, the center also promotes habitat
protection and provides public education on local conservation initiatives by managing a
radio station, visiting local communities, and has established a local school that can
accommodate up to 30 children. When gibbons arrive at Kalaweit they receive a full health
evaluation and spend one month in quarantine. They are tested for Hepatitis A, B, and C, as
well as Tuberculosis, and Herpes Simplex Virus 1 and 2. The enclosures at Kalaweit are
built into the surrounding vegetation, using trees as the main supports as much as possible.
The dimensions of the enclosures are approximately three by three by five meters. There are
two types of socialization enclosures in which gibbons are housed in, one for immature
25
gibbons and one for adult pairs. The socialization enclosures for immature gibbons may
contain several individuals at any one time. This is done in an attempt to facilitate the
socialization process among conspecifics. Rehabilitated gibbons are released based on
reintroduction guidelines established by the IUCN. The only published scientific data of
rehabilitated and reintroduced gibbons from Kalaweit are provided by Cheyne (2004) and
used for comparative analysis in this study. Presently, the released gibbons from Kalaweit
are alive and doing well on Mintin Island (S. Cheyne, personal communication, July 22,
2010).
THE JAVAN GIBBON RESCUE AND REHABILITATION
CENTER (JGC)
The Javan Gibbon Rescue and Rehabilitation Center, the site of this thesis research, is
the only center on Java that focuses solely on the rescue, rehabilitation, and reintroduction of
former pet Javan gibbons (Figure 2). When a gibbon first arrives at JGC they receive a full
health evaluation (they are tested for Hepatitis A, B, C, as well as Tuberculosis, and Herpes
Simplex Virus 1 and 2) and spend one month in quarantine. This is done in order to
determine the gibbon’s health status and ensure they can be properly treated if they do suffer
from any disease. In addition, this process provides the keepers with an opportunity to gain a
better understanding of the gibbon’s temperament. Once the gibbon is removed from
quarantine it is then moved into a socialization enclosure, ideally next to another gibbon of
the opposite sex. The socialization enclosure consists of two sections that are connected by a
small corridor. The keepers will then periodically open the corridor allowing the gibbons the
opportunity to interact with one another. After a period of observation (one month
minimum) the keepers will determine whether the gibbons will make a suitable pair based on
the level of affiliative interaction between the two gibbons. If this step in the socialization
process is successful and the keepers determine the gibbons will make a good pair, they are
then moved into larger, more naturalistic enclosures.
This thesis research represents the first systematic study conducted at JGC
documenting the pre- and post-release behavior of Javan gibbons during the rehabilitation
and reintroduction process.
26
Figure 2. Map illustrating the location of JGC (adapted from Andayani, N., Morales, J. C., Forstner, M. R. J., Supriatna, J., & Melnick, D. J. (2001). Genetic variability in mtDNA of the silvery gibbon: Implications for the conservation of a critically endangered species. Conservation Biology, 15, 770-775).
27
CHAPTER 3
RESEARCH QUESTION AND METHODS
This chapter comprises the research question and methods pertaining to my research.
RESEARCH QUESTION
Can wild-born, former pet Javan gibbons acquire the appropriate behavioral
repertoire necessary to survive in the wild after a period of rehabilitation?
To answer this question, I conducted behavioral observations (“pre-release”) on three
pairs of Javan gibbons that were deemed potential candidates for release by the staff of JGC
(staff based their decisions on preliminary, informal observations). I compare these results to
Cheyne et al.’s (2008) recommended behavioral criteria for rehabilitated gibbons. I also
conducted behavioral observations (“post-release”) on the first pair of Javan gibbons released
in October 2009, in order to determine whether the set of criteria is in fact a suitable measure
of the behavioral preparedness of gibbons in a rehabilitation program.
METHODS
This section provides details pertaining to the study site, study group, data collection
techniques and methods by which I analyzed the data.
Study Site
The pre-release observations were conducted at the Javan Gibbon Rescue and
Rehabilitation Center (106°50' - 107°02' E; 6°41' - 6°51' S) in Gunung Gede- Pangrango
National Park, West Java, Indonesia (Figure 2, p. 26). The park is one of the first five
National Parks established in Indonesia that were designated as UNESCO World Biosphere
Reserves and is comprised of two volcanoes, Mt. Gede and Mt. Pangrango. The park covers
a total area of 21, 975 hectares, with only 15, 196 hectares being less than 1,600 meters
above sea level, therefore, suitable habitat for Javan gibbons. It is located in one of the
wettest parts of West Java, with an average annual rainfall of 3,000 - 4,200 millimeters.
Gunung Gede-Pangrango National Park represents a unique diversity of ecosystems ranging
28
from lower montane/sub-montane, montane, sub-alpine forest, savanna, and marshland
(Supriatna, 2006). The park is home to an array of unique flora and fauna, several species of
which are considered endangered. Among those endangered include four primates: the Javan
gibbon, the grizzled leaf monkey (Surili) (Presbytis comate comate), the ebony leaf monkey
(Lutung) (Trachypithecus auratus auratus), and the Javan slow loris (Nycticebus javanicus).
There are also long-tailed macaques (Macaca fascicularis) living in the park, but they are
categorized as Least Concern (IUCN, 2010). Other endangered species living in the park
include the Javan leopard (Panthera pardus melas), dhole (Cuon alpinus javanicus), the
Javan Hawk- eagle (Spizaetus bartelsi) and the Javan Scops Owl (Otus angelinae) (IUCN,
2010; Supriatna, 2006).
All three of the pairs of gibbons included in this study were housed in naturalistic
enclosures. The naturalistic enclosures are built into the surrounding forest with trees being
used as the primary support (Figure 3). They range from six to eight meters high and vary in
their overall shape and width due to the natural landscape. Once the gibbons are moved into
these enclosures, there is very minimal human/gibbon interaction. The enclosures are placed
among trees so the gibbons have limited view of the neighboring enclosures. The gibbons
are fed a diverse diet that consists of fruit and vegetables from the local market, as well as
wild fruit and leaves collected from the forest in the park. In addition, they are also given a
variety of greens, tofu, tempe, peanuts, and sweet potatoes.
Post-release observations on one pair of Javan gibbons were conducted in an isolated
patch of forest (5 ha) located in Gunung Gede-Pangrango National Park, approximately two
kilometers away from JGC. The forest patch, Patiwel, is primary forest and is surrounded by
cropland (Figure 4). A pre-release survey was conducted to assess the quality of habitat at
the release site (Kurniyawati; in press). It was determined that there was adequate food
sources (i.e., a sufficient amount of fruit, leaves, flowers, and insects) to sustain a pair of
gibbons and potential offspring until a forest corridor has grown to a sufficient height which
will allow the gibbons to move into the larger forested area of the park.
Study Group
JGC is currently home to 31 Javan gibbons, 18 of which are housed in pairs. At the
time of this research, three pairs were deemed appropriate for release based on preliminary,
29
Figure 3. Naturalistic enclosure at Javan Gibbon Rescue and Rehabilitation Center.
30
Figure 4. Release site, Patiwel.
informal observations conducted by the staff of JGC. Each pair consisted of an adult female
and an adult male and were chosen based on certain behavioral criteria, namely observed
copulations and a cohesive pair-bond. If a pair of gibbons is observed grooming, playing,
copulating, or regularly maintaining close proximity to one another, they are considered to
have a cohesive pair-bond. All of the gibbons at JGC came from the illegal pet trade. They
were wild-born, human-raised and most likely had little to no previous contact with other
gibbons. All six of the gibbons included in this study are free of Tuberculosis and other
diseases (a necessary health criteria for release). The pairs observed in this study are: Septa
and Echi, Nancy and Jeffry, Kiki and Sadewa (Table 1).
DATA COLLECTION
This section provides details as to how I collected data for both the pre- and post-
release phases of the research.
31
Table 1. Description and Pair Names of the Study Gibbons
Gibbon Pair Estimated DOB Sex Origin Number of years paired Jeffry Nancy
1998 1998
M F
Jakarta Depok, West Java
5 years
Sadewa Kiki
2000 2000
M F
Sukabumi, West Java Sukabumi, West Java
2 years
Septa Echi
1999 1999
M F
Cianjur, West Java Sukabumi, West Java
2 years
Pre-Release
I observed each individual gibbon utilizing focal animal time-interval sampling and
ad libitum as necessary (Altmann, 1974). In order to generate an activity budget for each
pair of gibbons, I recorded the observed behavioral states at one-minute intervals for the
duration of the observation period (2-3 hrs) (Table 2). In addition to recording the behavioral
states at one-minute intervals, I recorded the gibbons’ proximity to one another, and their
location in the enclosure (see Data Analysis section for operational definitions). I collected a
total of 287 hours over 43 days (Jeffry and Nancy, 94hrs; Sadewa and Kiki, 93hrs; Septa and
Echi, 100hrs). Data were collected between the hours 0600h and 1700h. In an effort to
collect equivalent amounts of data for each pair of gibbons throughout the hours of the day, I
rotated the order of the pairs with which I began each day. Following the recommendations
of Cheyne et al. (2008), I recorded positive association behaviors including: grooming,
playing, and copulating. Other major activities included in the activity budget are:
feed/foraging, locomoting, vocalizing, and other. Human-directed behavior and stereotypic
behavior (see Ethogram, Table 2), were recorded as either events or states depending on the
duration of occurrence.
Post-Release
On Friday, October 16th, 2009, the staff of JGC, Conservation International-
Indonesia, Gunung- Gede Pangrango National Park, and members of the Indonesian
Department of Forestry, released Septa and Echi back into the wild (Figures 5 and 6). They
are the first pair of wild-born, former pet Javan gibbons to be reintroduced into the wild.
They were released into a small, isolated patch of forest, Patiwel, located within Gunung
Gede-Pangrango National Park.
32
Table 2. Ethogram for Javan Gibbon
Behavioral State
Locomote Brachiate Moving in a swinging motion by using one arm and then the other to
grab a substrate and move self in a forward direction. Climb Using arms to reach and pull body in an upward movement-can occur in
a sideways or downward direction when on the fence Bipedal Individual is walking/running on two legs in an upright position Rest Sitting or lying down while either asleep or awake Feed The act in which an individual physically consumes an edible item, also
includes drinking. Social Behavior Groom Process of cleaning or picking through partner’s hair using hands or
mouth. Copulate The pair is engaged in sexual intercourse. Play Individual is engaged in activity with another individual-usually consists
of wrestling or chasing each other. Agonistic Includes biting, slapping, pushing, or threatening another individual,
includes open-mouth threats. Vocalize Individual creates loud vocalization consisting of varying notes/pitches. Human-Directed Behavior
Any behavior directed at humans and is a result of human (keeper/observer) presence.
Stereotypic Behavior
Abnormal behavior resulting from captivity (i.e., rocking, twitching, repetitive swinging).
Other Self-groom, urinate, or defecate.
Adapted from Cheyne, S. M. (2004). Assessing rehabilitation and reintroduction of captive-raised gibbons in Indonesia. (Doctoral dissertation). University of Cambridge, Cambridge, England and Malone, N. M. (2007). The socioecology of the critically endangered Javan gibbon (Hylobates moloch); Assessing the impact of anthropogenic disturbance on primate social systems. (Doctoral dissertation) University of Oregon, Portland, OR.
33
Figure 5. Septa and Echi being released from their enclosure at (Photo by Duhe Anfield).
34
Figure 6. Septa after release (Photo by Duhe Anfield).
Since I was provided with the opportunity to conduct post-release behavioral
observations on the pair, my intended method for data collection consisted of conducting full
day focal follows utilizing time-interval sampling (Altmann, 1974). Unfortunately, due to
the unexpected aggression the gibbons exhibited towards human observers, I was unable to
systematically collect data. The gibbons made it very difficult to follow them and prevented
observers from monitoring them at a close distance. Therefore, I utilized ad-libitum
(Altmann, 1974) and opportunistically recorded any behavioral data I was able to collect,
whenever I could observe the gibbons from a safe distance. I collected 50 hours of ad-
libitum data over a total of 15 days. Data were collected between the hours of 0530 h and
1600h.
DATA ANALYSIS
In order to determine whether the gibbons were behaviorally ready for release, I
compared the average percent of observation records observed in major activities of the
Javan gibbons at JGC to the guidelines recommended by Cheyne et al. (2008) (Table 3). The
35
Table 3. Proposed Criteria for Assessing the Suitability of Gibbons for Release
Behavior of wild gibbons. Proposed criteria for assessing suitability of gibbons for release.
Gibbons spend 20–75% of travel time brachiating.
The gibbon should be able to move around the enclosure well, and most of this movement should be by brachiation
Gibbons spend 50–65% of their time in the upper and emergent canopy and have never been observed sleeping on the ground.
No more than 5% of time to be spent on the ground for any purpose. Gibbons should be at the top of the cage for at least 40% of the time and should not be sleeping on the ground at all
The cohesion of the pair is important for territorial defense and successful raising of offspring.
The pair should be spending at least 7% of total activity time in positive pair association. At least 3% of total activity time should be spent allogrooming.
Proof that the gibbons are sexually mature and active.
They should be copulating successfully and each member of the pair should be able to initiate successful copulation with the other.
To allow gibbons to survive once reintroduced their behavior should mimic that of wild gibbons.
Activity budgets should approximate those of wild conspecifics in all major categories i.e., feeding, resting and travelling
Stereotypic behaviors are a product of captivity.
No more than 3% of total activity time engaged in a severe stereotypic behavior e.g., rocking or self-harm.
Cheyne, S. M., Chivers, D. J., & Sugardjito, J. (2008). Biology and behavior of reintroduced gibbons. Biodiversity Conservation, 17, 1741-1751.
table highlights the important aspects of wild gibbon behavior that a rehabilitated pair should
master before they are reintroduced. It is also crucial that the gibbons solicit little to no
human interaction (i.e., soliciting grooming from keepers, begging, aggressive behavior)
because once they are released this behavior could potentially be harmful to the gibbons or to
humans, and hinder their chance for survival in the wild.
I calculated the average percent of observation records each of the pairs of gibbons
spent in major activities (rest, feed/forage, social, locomotion, and vocalization) during the
observation period. These data were then compared to the activity budgets from the gibbons
in the rehabilitation program at Kalaweit gibbon center in Central Kalimantan, Indonesia
(Cheyne et al., 2008). Cheyne (2004) conducted behavioral observations utilizing ten-minute
scan sampling on each individual gibbon. In addition, in order to determine how the
gibbons’ behavior at JGC compares to wild conspecifics, I compare my results to the activity
budgets of wild Javan gibbons living in Cagar Alam Leuweung Sancang (CALS) West, Java
36
obtained by Malone (2007). Malone (2007) utilized two-minute scan sampling in order to
determine the behavioral state for each individual gibbon in a focal group.
Based on the criteria established by Cheyne et al. (2008) (Table 3, p. 35),
rehabilitated gibbons should be spending at least seven percent of total activity time engaged
in social behavior, particularly positive association. Positive association behavior includes
any behavior where the two gibbons are interacting with one another in an affiliative manner,
such as grooming, copulating, and playing. Cheyne (2004) also suggests that the gibbons
should be spending at least three percent of total social time engaged in allo-grooming.
Therefore, I determined the percent of observation records each pair of gibbons spent
engaged in positive association. There was an observable change in Septa and Echi’s social
behavior once they were relocated to the acclimation enclosure at Patiwel. In order to test
this, I transformed the percentage of observation records for social behavior to obtain the
arcsine values. I then used a paired-sample t-test to determine whether there was a
significant difference in mean time spent in social behavior at JGC and Patiwel. Finally, I
report any instances of agonistic behavior as the number of occurrences out of total observed
behaviors.
Stereotypic behavior is considered to be abnormal behavior and is typically repetitive
and unchanging with no obvious goal or function. It has long been recognized that captivity
can result in behavioral and psychological problems in animals (Cheyne, 2004; Mason,
1991). Stereotypic behavior can arise from a failure to replicate essential environmental
requirements in the captive setting. There is much evidence of the detrimental effect of
removing young primates (and humans) from their mothers; the infants fail to develop social
skills and suffer psychologically from the separation (Harlow & Suomi, 1971; Mootnick &
Nadler, 1997). This abnormal behavior often arises because animals in captivity are not
given the opportunity to engage in natural behaviors and it can manifest itself in several
ways. According to Cheyne (2004), stereotypic behavior in gibbons is most commonly
expressed in one (or all) of the following forms: rocking (this may be in a loose body posture
or where the body is locked into a rigid position, often where the gibbon wraps its arms
around its upper torso); twitching (the gibbon does not perform smooth body movements,
instead it jerks its limbs and head when it wishes to move them); repetitive swinging on a
fixed substrate (the gibbon was not traveling around the cage); repetitive brachiation along
37
the same route (the gibbon was traveling and not making play-face); self harm (biting the
skin, hitting limbs against the enclosure); and teeth scraping (scraping of canines along a
substrate). According to Cheyne et al. (2008) rehabilitated gibbons should not exceed three
percent of their total activity time engaged in severe stereotypic behavior (i.e., rocking or self
harm). If the gibbon was engaged in a form of stereotypic behavior that occurred for a
duration of longer than one-minute (i.e., individual is repetively brachiating along the same
route or repetivitely swinging on a substrate), it was recorded and included in the activity
budget. However, if the behavior occured instantaneously (i.e., teeth scraping), I report it as
an event.
In addition to stereotypic behavior, I included another category: human-directed
behavior. This category consists of behavior that is directed at humans and/or a result of
human (keeper/observer) presence. Gibbons which were separated from their mothers at a
very young age and who have lived a significant portion of their lives with humans may
potentially develop an abnormal behavioral repertoire (Mootnick & Nadler, 1997). When the
gibbons are then placed into a more appropriate social environment (i.e., with conspecifics)
in a rescue/rehabilitation center, the abnormal behavior may persist causing the gibbon to act
aggressively towards humans or their mate, or conversely, remain tame and seek affection
from human caretakers (personal observation). Former pet gibbons will most likely never
have had the opportunity to interact with other gibbons once they were removed from the
wild (Cheyne, 2004). Upon reaching sexual maturity, former pet gibbons may exhibit
aggressive behavior or engage in other forms of abnormal behavior directed at humans
(Cheyne, 2004; Mootnick & Nadler, 1997). The most commonly observed behavior is
“posterior display” (Cheyne, 2004) or referred to as “hostile presenting” (Mootnick et al.,
2006). This behavior resembles a sexual presentation by the female when she presents her
rump to a male, but is also observed in male gibbons (Cheyne, 2004; Mootnick, Baker,
Nadler, & Merker, 2006). This behavior has been observed in captive gibbons at the Javan
Gibbon Center, Kalaweit gibbon center, and the Gibbon Conservation Center in Santa
Clarita, California (Cheyne, 2004; Mootnick et al., 2006). Other human directed behavior
includes solicitation for attention or grooming from humans, begging for food, or other
unusual behavior that does not fit into the category of stereotypy. For example, one of the
female gibbons included in this study, Echi, would often lock herself in the shift cage while I
38
was observing the pair. This behavior would consist of her sitting in the shift cage,
attempting to unattach the rope that held up the shift gate, and if she was successful, the gate
would slam closed separating her from Septa (the male) and the larger part of the enclosure.
I would then have to stop conducting observations and go open the shift gate. I would
typically have to use a long stick to retrieve the rope and grab the string so I could attach it
back to the nail holding it in place. Echi would then take that opportunity to aggressively
grab at me and the substrate I was using to assist me. None of the keepers at JGC had ever
observed her do this before. Sometimes this behavior would go on for longer than a few
minutes, therefore, I included it in my time-interval sampling of behavioral states. All other
forms of human-directed behavior typically occurred instantaneously and I report them as
events.
The enclosures at JGC are all built into the surrounding forest with trees as the
primary support. As a result, they vary in their overall shape and size, averaging eight to ten
meters in width. They also range between six to eight meters high. It was not entirely
possible for the gibbons to be too far from one another within the enclosure. However, it was
still necessary to record the estimated distance between the two gibbons. Close-proximity
(within one meter), or “sitting together”, without allogrooming or copulating, is considered to
be one of the primary social interactions of adult gibbons (Palombit, 1994). This information
can potentially be used as a good indicator to the cohesiveness of the pair bond and possibly
provide insight as to whether the pair will survive the transition from captivity to living in the
wild (Cheyne et al., 2008). For purposes of recording the maintenance of proximity between
the pair of gibbons, I developed codes representing the estimated distance in meters and
recorded the distance at the one-minute interval. The codes are as follows: contact (0);
within one meter (1); one to three meters (2); three to five meters (3); and greater than five
meters (which would indicate the gibbons were on complete opposite sides of their
enclosure) (4). I report the percent of observation records the gibbons spent at different
measures of proximity.
Gibbons that have spent the majority of their life in captivity as pets may have had
very little opportunity to climb or spend time high off the ground (some owners may tie their
pet gibbon to a tree). Wild gibbons will rarely come down to the forest floor due to higher
risk of predation from leopards, pythons, and humans. Instead they typically spend the
39
majority of their time at 25 meters or higher in the forest canopy (Bartlett, 2007; Cheyne,
2004). Former pet gibbons are very comfortable spending time at the ground level of their
enclosures due to the fact they most likely lived in a small cage on the ground and did not
have the opportunity to climb or establish the coordination necessary for a life in the canopy
(Cheyne, 2004). In addition, captive gibbons will often come to the ground level of their
enclosure to retrieve fallen food or to forage, tending not to spend more than a few minutes
there. If the gibbons are to survive successfully in the wild, it is crucial that they remain at
the upper level of the canopy. This criterion can be used as another measure of the gibbons’
suitability for release into the wild (Cheyne, 2004). The canopy level measurements were
used to determine at what level within the enclosure the gibbons are spending most of their
time. The canopy level measurements were estimated and recorded using the following
codes: ground (0); one to three meters (1); three to five meters (2); and upper level of
enclosure (3). Cheyne (2004) recommends that rehabilitated gibbons should not be spending
more than five percent of their time on the ground and should remain in the upper level of the
enclosure at least 40 percent of the time. I report the percent of observation records that each
gibbon spent in the location of their enclosure.
Gibbons that have been kept as pets most likely never learned how to locomote
appropriately, such as brachiate, leap or climb, because they probably were kept in cages that
did not allow them to practice and develop proper forms of locomotion. In addition, they
may also have poorly-developed muscles and will lack the coordination necessary for a life
in the forest canopy (Cheyne, 2004). According to Cheyne (2004), rehabilitated gibbons
should be able to traverse around their enclosure efficiently and should utilize brachiation as
the primary mode of travel. The gibbons at JGC are able to brachiate efficiently around their
enclosures by either holding onto the bamboo supports at the top of the enclosure, the
chainlength fence, or there are several substrates placed throughout the upper level which
they can also hold onto. In order to determine which form of locomotion the gibbons prefer
to use for traveling around their enclosure, I calculated the percent of total observation
records that the gibbons spent engaged in each form of locomotion (i.e., climbing,
brachiating, walking bipedally).
40
CHAPTER 4
RESULTS
This section details the results including a between-site comparison, within-site
comparisons, a comparison of Cheyne et al.’s (2008) criteria, and the post-release behavioral
results.
BETWEEN-SITE COMPARISON
The comparison between-sites include activity budget data from: Javan Gibbon
Center, Kalaweit, and Cagar Alam Leuweung Sancang. The results from the three different
sites may represent a crude comparison due to slightly different sampling methods utilized in
each study (i.e., duration of time-interval sampling). However, considering the difference is
minimal, I am fairly confident the results can still be meaningfully compared. When the
results of the levels of activity are compared across the three groups of gibbons at different
sites, the gibbons at JGC spent more time resting than either of the two other groups at CALS
and Kalaweit gibbon center (KP) (71% vs. 15.2% & 50% respectively) (Figure 7). The wild
Javan gibbons in CALS spent the majority of their time feeding (foraging is included in the
feed category for all three groups) at 50.6 percent compared to the 18 percent of time the
gibbons at KP spent feeding, and the gibbons at JGC spent the least amount of time feeding
and foraging at 15.8 percent. The gibbons at KP had the highest percentage of vocalizing,
followed by the gibbons at CALS, with the gibbons at JGC spending the least amount of time
vocalizing. The three groups of gibbons followed a similar pattern with regards to social
behavior and locomotion, with the KP gibbons having the highest percentages (8% and 19%
respectively), followed by CALS(6.7% and 13.3% ) and JGC (2.2% and 6.7%). However,
the percent of observation records that all three pairs spent in social behavior is relatively low
when compared with the other two groups of gibbons at KP and CALS.
41
Figure 7. Average percent of observation records gibbons spent in activity at JGC, KP, and CALS. Data represent combined percentage of male and female gibbons in each activity. JGC-6 individuals (3 adult males; 3 adult females); CALS-5 individuals (3 adult males; 2 adult females); and KP-41 individuals (number of each sex is unknown).
WITHIN-SITE COMPARISONS
The within-site comparisons involve all analyses from the Javan gibbons included in
this study and an evaluation of how their behavior compares to the guidelines established by
Cheyne et al. (2008).
Activity Budget
Figure 8 illustrates the average percent of observation records that each pair of
gibbons from JGC spent engaged in particular activities. When the results are compared
across the three pairs of gibbons included in this study, the results show similar activity
patterns. Septa and Echi had the highest percentage of resting at 74.5 percent, whereas
Jeffry and Nancy had the lowest (66.3 %). Jeffry and Nancy had the highest percent of
feeding/foraging at 21.1 percent and Sadewa and Kiki and Septa and Echi spent less time
feeding and foraging (12.5 % and 13.7% respectively). Sadewa and Kiki engaged in the
most social behavior at 2.85 percent and the other two pairs, Septa and Echi and Jeffry and
Nancy spent roughly the same percent of observation records as each other engaged in social
behavior (1.89% and 1.85%). For activity budget purposes, any stereotypic and human-
42
Figure 8. Percent of observation records the gibbons spent in different activities.
directed behavior that occurred for duration of longer than one-minute was included in the
other category; therefore, Sadewa and Kiki had the highest percentage at 7.1 percent because
they often displayed stereotypic behavior (repetitive brachiation and swinging on substrates),
followed by Jeffry and Nancy (5.06%), and Septa and Echi (2.6%).
Social Behavior
Jeffry and Nancy spent 1.36 percent of their total percent of observation records
engaged in grooming and 0.46 percent of total percent of observation records playing. There
were no observed instances of copulation during the sample period (94 hrs). There were only
four instances of agnostic behavior observed out of a total of 5,652 observed behaviors. The
agonistic behavior typically involved a “swat” or quick grab.
43
Sadewa and Kiki spent a very small percent of observation records engaged in
positive pair association (2.85%). The pair only spent 0.35 percent of total observation
records engaged in allo-grooming. However, they engaged in play behavior for 2.5 percent
of the time. Although Kiki has been pregnant in the past (JGC staff, personal
communication, June 2008) indicating the pair are sexually active, I did not observe any
instances of copulation during the entire observation period (93 hrs). There were no
observed instances of agonistic behavior between Sadewa and Kiki.
Septa and Echi spent a total of 1.9 percent, also very small, of observation records
engaged in positive pair association behavior. They were observed grooming 1.24 percent of
total time. Echi has also been pregnant in the past (JGC staff, personal communication, June
2008) and they were observed copulating four times during the observation period (100 hrs).
There was a significant increase in the pair’s social behavior once they were moved over to
the acclimation enclosure at Patiwel (paired samples t-test; t= 2.185 df=16, p= 0.012). All of
the play behavior observed and all four instances of copulation occurred at Patiwel. There
was one instance of agonistic behavior observed out of 5,982 total observed behaviors (at
JGC). The incident occurred after the keepers let Echi out of the shift cage (after she had
locked herself in) and as soon as she made her way to the top of the enclosure, Septa
physically attacked her causing her to fall to the bottom of the enclosure.
Stereotypic Behavior
Jeffry and Nancy spent a total of 4.8 percent of observation records engaged in
stereotypic behavior. The nature of their stereotypic behavior consisted of repetitive
swinging on a substrate followed by bouncing up and down on top of the substrate (Jeffry,
3.5% ), and repetitve brachiation around the enclosure (Nancy, 1.3%). Nancy would also
occasionally bounce up and down on top of a substrate, albeit far less than Jeffry. The onset
of behavior typically occurred before feeding times. However, Jeffry would regularly swing
repetitively on one particular substrate at times other than before feeding.
Sadewa and Kiki spent a considerable amount of total observation records engaged in
stereotypic behavior at 7.1 percent. Both gibbons would repetitively brachiate around the
enclosure and swing repetitively on substrates. This behavior was observed most frequently
before feeding, but would also occur before play bouts. If the gibbons were engaged in play
44
behavior and noticabley chasing one another while brachiating around their enclosure, this
was not considered stereotypic behavior.
Septa and Echi had the lowest percentage of stereotypic behavior of all three of the
pairs at 0.71 percent. The most observed stereotypic behavior exhibited by Septa was teeth
scraping (two occurences out of 5,962 total observation records). He would typically display
this behavior upon my arrival at the enclosure for observation and would usually only do it
one time. Both gibbons would occasionally brachiate around the enclosure before feeding
time.
Human-Directed Behavior
Jeffry, who is considered a relatively tame gibbon by JGC staff, did not exhibit any
human-directed behavior, affiliative or aggressive. Nancy had one instance of human-
directed behavior out of a total of 5,652 total observed behaviors and it was an open-mouth
threat directed at me.
Sadewa and Kiki only had three instances (one and two respectively) out of a total of
5,576 total observation records. Sadewa attempted to grab a keeper while he was placing
food in the feed basket and Kiki would present her backside to the keepers (or me if I
approached the enclosure).
Septa and Echi were quite aggressive and would regularly attempt to grab the keepers
while they were feeding them. There were only 12 observed instances of human-directed
behavior out of a total of 5,962 total observation records for Septa. In addition to the unusual
behavior of Echi locking herself in the shift cage while I was observing them, she would also
present her backside to both the keepers and myself, if we got too close to the enclosure. Out
of a total of 5,962 observation records for Echi, 58 were directed at humans. Compared to
the other five gibbons Echi displayed the most human-directed behavior.
Maintenance of Proximity
Figure 9 illustrates the percent observation records that each of the pairs of gibbons
spent at different estimated measures of proximity. Jeffry and Nancy spent the majority of
the observation records (82.2%) at a distance of at least one meter apart, but rarely spent time
more than five meters away from one another. However, the percent of observation records
they spent in actual contact was relatively low (6.1%).
45
Figure 9. Percent of observation records gibbons spent in proximity.
Sadewa and Kiki spent the majority of the observation records (86.9%) at a distance
of at least one meter apart. They also rarely were more than five meters away from one
another. The amount of observation records they spent in actual contact was low at 3.5
percent.
Septa and Echi spent the majority of the observation records (76.7%) within relatively
close proximity, usually maintaining a distance of within three meters of one another. Unlike
the other two pairs, Septa and Echi spent a considerable amount of observation records
(15.7%) in direct contact with each other; they would often sit right next to one another on
top of their sleeping box.
Location in Enclosure
Figure 10 illustrates the percent of observation records that each of the pairs of
gibbons spent in different levels of their enclosures.
Jeffry and Nancy would occasionally forage for food at the lower level (1-3 m) of the
enclosure (27.1% and 22.7% respectively), as well as coming down to the floor of the
46
Figure 10. Percent of observation records spent in location of enclosure.
enclosure (0.16% for both), but would rarely spend more than a few minutes there. Both
gibbons spent the majority of the observation records at a level greater than three meters
above the ground: Jeffry (72.6%) and Nancy 77.1 percent.
Sadewa would often come down to the lower level of the enclosure (12.2%), not only
to forage, but also just to sit on the floor of the enclosure. He still remained in the mid to
upper level (> 3 m) of the enclosure for the majority of the observation records (87.9%). In
comparison, Kiki rarely ever came down to the lower levels of the enclosure and spent 91
percent of her time in the upper level. The only instances where she came down to the
ground (.07%) were to play with Sadewa while he was sitting on the floor of the enclosure.
Septa and Echi spent most of their time at the upper level of their enclosure at 67.9
percent and 68.5 percent respectively. Both gibbons spent a minimal amount of time at the
lower level of the enclosure (Septa 11.03% and Echi 12.1%). Echi was never observed to
come down to the floor of the enclosure, and Septa would only come down to retrieve fallen
food or occasionally to forage.
47
Locomotion
Figure 11 illustrates the percent of observation records that Jeffry and Nancy spent in
different modes of locomotion. Jeffry prefered to climb around the enclosure more than
brachiate (67% vs. 25%). Nancy utilized brachiation as her main form of travel around the
enclosure (63%). She also had a very peculiar way of walking bipedally; she would bop up
and down as she walked across a subsrate. However, this did not impede upon her ability to
walk bipedally.
Figure 11. Percent of observation records gibbons spent in different modes of locomotion.
Figure 12 illustrates the percent of observation records that Sadewa and Kiki spent in
different modes of locomotion. Both gibbons utilized brachiation as their primary mode of
travel around the enclosure (Sadewa ,94% and Kiki, 87%). Kiki had a slightly higher
percentage of climbing than Sadewa (13% vs. 5%).
Figure 13 illustrates the percent of observation records that Septa and Echi spent in
different modes of locomotion. Septa and Echi both utilized brachiation as their primary
mode of travel around the enclosure (93% and 80% respectively). Echi also had a higher
48
Figure 12. Percent of observation records gibbons spent in different modes of locomotion.
Figure 13. Percent of observation records gibbons spent in different modes of locomotion.
49
percentage of climb than Septa (15% vs. 6%). Out of all six gibbons, Echi had the highest
percentage of walking bipedally at five percent.
Evaluation of Recommended Behavioral Criteria
Table 4 presents a summary of all the necessary behavioral criteria established by
Cheyne et al. (2008), in which rehabilitated gibbons should meet prior to their release, and
whether the Javan gibbons included in this study satisfied the criteria or not.
Table 4. Summary of Behavioral Criteria for Pre-Release Observations
Proposed criteria for assessing suitability of gibbons for release*
Jeffry/Nancy
Sadewa/Kiki
Septa/Echi
The gibbon should be able to move around the enclosure well, and most of this movement should be by brachiation.
Jeffry
Nancy
Sadewa
Kiki
Septa
Echi
No more than 5% of time to be spent on the ground for any purpose. Gibbons should be at the top of the cage for at least 40% of the time and should not be sleeping on the ground at all.
Jeffry
Nancy
Sadewa
Kiki
Septa
Echi
The pair should be spending at least 7% of total activity time in positive pair association. At least 3% of total activity time should be spent allogrooming.
They should be copulating successfully and each member of the pair should be able to initiate successful copulation with the other.
Activity budgets should approximate those of wild conspecifics in all major categories i.e., feeding, resting and travelling.
No more than 3% of total activity time engaged in a severe stereotypic behavior e.g., rocking or self-harm.
= did not satisfy criteria = satisfied criteria * Cheyne, S. M., Chivers, D. J., & Sugardjito, J. (2008). Biology and behavior of reintroduced gibbons. Biodiversity Conservation, 17, 1741-1751.
RELEASE OF JAVAN GIBBON PAIR
Septa and Echi (Figure 14) demonstrated a high level of territoriality directed both at
us and the monkeys who also inhabit Patiwel. There are approximately seven ebony langurs
50
Figure 14. Septa and Echi post-release (Photo by Anton Ario/CI-Indonesia).
51
that have been observed living in the small forest, and groups of long-tailed macaques
frequently come into Patiwel from the larger area of the park to forage and feed. When the
gibbons encountered either one of the groups of monkeys, they would engage in territorial
behavior that is typical of wild Javan gibbons. In the wild, both male and female gibbons
will actively defend their territory by approaching the intruder (usually the male initiates the
approach), performing visual displays (branch shaking, brachiating) (Malone, 2007), and
males may produce solo calls while the female sings. Territorial disputes rarely involve
physical aggression in the wild. Wild gibbons regularly tolerate food competitors, such as
macaques and other primates (MacKinnon, 1977), but unlike captive gibbons, wild gibbons
have been exposed to the various other species in the forest from a very young age and know
which species are potential threats and which are not (Cheyne, 2004). There were only two
observed interactions between the gibbons and the monkeys and consisted of the gibbons
displaying, chasing the monkeys, and vocalizing. On one occasion the gibbons were
displaced by the long-tailed macaques and moved further west into the forest, and the other
encounter resulted in the long-tailed macaques being displaced and moving to the west side
of Patiwel.
Both Septa and Echi exhibited a very high level of aggression towards myself and
observers from JGC as we attempted to monitor them in the forest. They directed their
territorial/aggressive behavior towards us, similarly to how they responded to the monkeys in
Patiwel. A typical observational occurrence consisted of us attempting to locate them in the
forest and once they became aware of us, they approached us, coming very low (< 5 m) in the
canopy to threaten us. One incident resulted in one of the observers being physically
attacked and bitten by the pair of gibbons. On our first day of observations when we located
the gibbons in Patiwel, they were feeding on the fruit of a fallen tree just off the trail. After
approximately 2 hours and 20 minutes into our observation, Septa moved into a tree that
branched out above the trail about 25 meters up from where myself and the other observer
were standing. After a few minutes, Septa dropped down onto the trail and began to
approach us, walking bipedally down the trail. Igud and I quickly retreated out of the forest
and returned to our post located just outside of Patiwel. We decided to continue our
observations of the pair at 13:00 and returned to the forest. When we located the gibbons we
found them in the same general area as earlier that morning. Once again, Septa dropped
52
down onto the forest trail and began to approach me (I was standing directly on the trail and
Igud was standing to the far right of the trail in the forest). This time I moved toward Septa
in an attempt to frighten him away and he turned and went the other direction, again walking
bipedally up the forest trail. I was not aware of Echi’s location at the time. After a few
minutes, Igud and I continued up the forest trail. When we reached the top of the trail where
the gibbons’ old enclosure is located, we found both Septa and Echi foraging around the
vicinity. Once they became aware of us, Septa moved down onto the ground at the top of the
trail and Echi began moving towards us through the trees. Igud and I were approximately
25-30 meters down the forest trail from the enclosure. Echi began to move closer to us,
brachiating through the lower level of the canopy, while Septa ran bipedally down the forest
trail and they proceeded to chase us. Unfortunately, both Igud and I tripped and fell down
the trail. Igud fell first and then I, so at that moment both of the gibbons came upon Igud
first and physically attacked and bit him. We were able to get up and continue running out of
the forest. Septa and Echi followed us all of the way out and moved into the upper level of
the canopy. Echi then began to vocalize and display at the forest edge. Fortunately, there
were no other incidents resulting in aggressive physical contact, but Septa and Echi
continued to chase us out of the forest each and every time we attempted to observe them.
They displayed similar behavior to the first incident. Once they became aware of us, they
approached us coming very low into the canopy, sometimes within five meters of the ground,
and threatened us. We would use small (~ 3 meters) bamboo sticks to bang against the trees
in an attempt to frighten the gibbons back up into the canopy of the forest. This was always
unsuccessful. The guys would also use a sling-shot and fire small pieces of fruit in the
gibbons’ direction (never actually hitting Septa or Echi), also attempting to frighten them
away from us. This was also unsuccessful. Septa and Echi were completely fearless of
human observers. Fortunately, we were able to observe the pair from the outer edge of the
forest, the surrounding fields, and the research station located just outside of the forests’
edge.
Aside from their aggressive behavior towards humans, they responded well to their
new forest environment from an ecological perspective. In addition to jointly defending their
territory, Septa and Echi often maintained a close proximity (typically within 5 meters) to
one another while foraging and feeding and were observed grooming each other as well as
53
copulating (by staff of JGC). We did not provide food supplementation for the gibbons.
Only on one occasion did we observe Septa and Echi foraging around their old enclosure (the
day of the attack) and not once did they seek us out for food or appear to be waiting around
their enclosure for food. While in captivity both gibbons showed a preference for wild
fruit/foliage that the keepers incorporated into their diet. From the first day they were
released, Septa and Echi were successful at locating the appropriate food sources (fruit,
leaves, flowers, and insects) in Patiwel and were observed drinking water from a hollow in a
tree.
Unfortunately, we were never able to locate them in the later part of the afternoon
when they would normally move into their sleeping tree, thus it is uncertain if they were
sleeping in the same tree. However, when we were able to locate them during Echi’s
morning call, both gibbons would be in the same tree, typically a Rasamala tree (Altingia
excelsa). Rasamala trees typically reach heights of 40-60 meters and with their crown
extending above the canopy, this provided Echi with the ideal setting for producing her
morning call. Both Septa and Echi utilized brachiation as their primary mode of locomotion
in Patiwel, in addition to leaping through the canopy. I would regularly observe them
leaping distances of greater than ten meters, which was spectacular. They would also climb
up vines or the bough of a tree to gain access to fruit patches within a tree and were also
observed foraging and feeding on smaller branches.
During the month of June 2010, I returned to Patiwel to assess the behavior of Septa
and Echi eight months post-release. Due to time constraints, I was only able to visit the
release site for one afternoon. Considering my experience with Septa and Echi while
attempting to conduct behavioral observations on them in the past, I did not enter the forest.
Instead, I walked around the edge of Patiwel until I could locate and see the gibbons from the
surrounding cropland. They were observed foraging and feeding in the trees at the edge of
the forest, and I was able to observe the pair grooming one another. Interestingly, once they
became aware of my presence, they exhibited the same aggressive behavior as before. Both
gibbons came to the edge of the forest and Echi began to vocalize and display in the upper
canopy. The staff of JGC, who continues to monitor Septa and Echi at Patiwel, have also
reported the gibbons are still acting territorial and aggressive towards them.
54
CHAPTER 5
DISCUSSION
In this study, I examined whether select pairs of Javan gibbons housed at the Javan
Gibbon Rescue and Rehabilitation Center have developed the appropriate behavioral
repertoire necessary to be reintroduced and survive in the wild. I interpret my results based
on other comparative studies of behavioral and ecological characteristics of the Hylobatidae.
I first discuss the general behavioral patterns observed for the Javan gibbons at JGC, as well
as comparing and contrasting the results to the activity budgets of white-bearded gibbons and
Mueller’s gibbons at Kalaweit gibbon center in Central Kalimantan, and wild Javan gibbons
at CALS in West Java. In addition, I further discuss and interpret my observations with
particular regards to social behavior, stereotypic and human-directed behavior, maintenance
of proximity, location in enclosure, and locomotion in more detail. I then provide a
discussion as to how the gibbons’ behavior at JGC compare to the criteria recommended by
Cheyne et al. (2008) and lastly, I discuss the conservation implications that are relevant to
this study.
BEHAVIORAL PATTERNS
All three pairs of gibbons included in this study at JGC spent the majority of the
observation records resting (average 71.1%). A relatively high percentage of time spent
resting is likely unavoidable considering the gibbons are confined to their enclosures and
have food provided for them. The gibbons at Kalaweit gibbon center spent approximately 50
percent of their observed time resting (Cheyne, 2004) and wild Javan gibbons at CALS spent
only an average of 15.2 percent observed time resting, both of which are considerably lower
than the Javan gibbons at JGC. I cannot account for the difference in time resting for the two
groups of gibbons in rehabilitation programs (JGC and Kalaweit), but the low percentage of
time resting for wild Javan gibbons seems more appropriate considering they spend more
time foraging and feeding. In fact, the majority of time observed for the wild Javan gibbons
at CALS was foraging and feeding at 50.6 percent. When gibbons are not traveling through
55
the forest, they tend to spend the majority of their time resting and feeding (Bartlett, 2003;
Srikosamatara & Brockelman, 1987). Similar to the Kalaweit gibbon center, the enclosures
at JGC are built into the surrounding forest. This environment allows the gibbons the
opportunity to engage in forms of enrichment and stimulation from the natural environment
by foraging for leaves and insects (Cheyne, 2004). The gibbons at Kalaweit spent 18 percent
of the time feeding and foraging and the gibbons at JGC spent only 15.8 percent of their
observed time feeding and foraging. Again, this is most likely due to the fact that their food
is provisioned for them and they do not have to search for it. However, it should be noted
that the gibbons at Kalaweit spent 19 percent of their observed time traveling, or engaged in
particular modes of locomotion. This is higher than both of the groups of Javan gibbons at
JGC and CALS (6.7% and 13.3% respectively). The issue of being confined to an enclosure
would not explain the difference in percentages for the two groups of gibbons at JGC and
Kalaweit.
Javan gibbons are one of only two species of gibbons that do not produce duet
vocalizations (Kloss’s gibbon being the other). While female Javan gibbons are considered
the vocal representative of the family, however, males will sometimes contribute to the
female’s great call and vocalize during territorial disputes (Geissmann, 2002). The average
percent of observation records that the Javan gibbons spent vocalizing was only .71 percent.
This is considerably lower than both of the other gibbon pairs at CALS and Kalaweit. The
wild Javan gibbons spent approximately 3.3 percent of the observed time vocalizing, whereas
the gibbons at Kalaweit spent an average of 6 percent observed time vocalizing. Both of the
species at Kalaweit, white-bearded gibbons and Mueller’s gibbons, do engage in duet
vocalizations, so this could explain why the gibbons at Kalaweit have a higher rate of
vocalizing than either of the groups of Javan gibbons. The wild Javan gibbons at CALS
consist of two neighboring groups and both male and female gibbons will attempt to defend
their territories by engaging in physical displays and vocalizing (Nijman, 2004, 2006;
Supriatna, 2006). Therefore, the wild Javan gibbons’ higher rate of vocalizing could be due
to the gibbons’ engaging in vocal territorial disputes on a regular basis. While the gibbons at
JGC are in enclosures that are hidden amongst trees and not entirely visible to one another,
they are still relatively close to each other. It is possible that not all of the gibbons at JGC are
vocalizing in the same social context as wild gibbons (i.e., not needing to physically defend
56
their territories from each other). The gibbons at JGC would often begin vocalizing upon
hearing wild Javan gibbons in the park vocalizing. Also, the gibbons at JGC typically
vocalize in the early morning hours (~0700-0900), so not all vocalizations were included in
my observation sample times. This issue could also potentially explain why the wild Javan
gibbons have a higher percentage of vocalizing than the Javan gibbons at JGC.
SOCIAL BEHAVIOR
Not one of the pairs of Javan gibbons at JGC satisfied the recommendation of at least
7 percent of total activity time in positive pair association. Gibbons have traditionally been
believed to engage in very little affiliative social behavior in comparison to other primates
(Bartlett, 2003; Leighton, 1987). After an extensive review of field studies on gibbon
behavior and ecology, Leighton (1987) concluded that: “gibbons spend surprisingly little
time socializing with one another. Singing takes up about 4 percent of the activity period,
grooming and social play usually less. The lack of social partners may contribute to such
low levels of interaction” (p. 137). The rates of social behavior observed in gibbons are
dependent on the number of available social partners, and since gibbons live in smaller
family units, this could explain why there are lower rates of social behavior observed in the
wild (Bartlett, 2003). It should be noted that Cheyne (2004) included juveniles and subadults
in her study, which is potentially the reason why the gibbons at Kalaweit had a relatively
higher average of time spent in social behavior (8%). Juvenile and subadult gibbons are
primarily responsible for the majority of social behavior observed in wild gibbons, and for
initiating social behavior with the adult gibbons (Bartlett, 2003). The wild Javan gibbons at
CALS had an average of 6.7 percent observed time spent in social behavior. This is
relatively higher than the Javan gibbon groups at JGC and could also be due to the younger
gibbons in the groups at CALS. There was a subadult female in one of the groups of wild
Javan gibbons at CALS, and both adult females in the groups had infants.
Studies of three species of gibbons; agile gibbons (Hylobates agilis) (Gittins &
Raemaekers, 1980), Kloss’s gibbons (Whitten, 1984), and Mueller’s gibbons (Leighton,
1987) determined that social activity was virtually absent all together (Bartlett, 2003).
Whereas other field studies with lar gibbons have yielded different results. One study at
Tanjong Triang in Malaysia (Ellefson, 1974), lar gibbons spent ~2-3 percent of the time
57
engaged in social activity and Gittins and Raemaekers (1980) reported that lar gibbons at
Kuala Lompat spent an average of 3 percent of their time engaged in social activity. Javan
gibbons at JGC show similar results, yielding an average of 2.2 percent of total time
observed in social behavior. Regardless, the display of grooming, playing, and copulation
suggest a well bonded pair (Brockelman, 1984; Cheyne, 2004). Playing and grooming were
the two most commonly observed behaviors. Septa and Echi was the only pair I observed
copulating during my study. This has significance considering the breeding pair of Javan
gibbons at the Perth Zoo in Australia has only been observed copulating two times (H.
Thompson, personal communication, October 7, 2009). It was interesting to observe how
Septa and Echi engaged in more social behavior (play, grooming, and copulation) once they
were moved over to Patiwel. I had never observed them play while they were living at JGC.
They also spent more time in close proximity and all four instances of copulation were
observed at Patiwel. Close-proximity (within one meter), or “sitting together”, without
allogrooming or copulating, is considered to be one of the primary social interactions of adult
gibbons (Palombit, 1994). The increase in social behavior may have been due to the fact that
there were not any other gibbons in the area and there was far less of a human presence at
Patiwel. Their overall behavioral profile would seem to indicate a cohesive pair-bond, which
will be beneficial for their survival in the wild. Wild gibbons will not necessarily feed,
forage, or even sleep in the same tree, but for this study measuring how much time the
gibbons spend in close proximity was useful for learning more about the nature of the pair-
bond in captivity (Nijman, 2004). For example, there was one pair of Javan gibbons at JGC
(not included in this study) that would not even sit on the same side of the enclosure as one
another. The male would sit inside their sleeping box or swing underneath it. The female
would move around the enclosure more, but anytime she came within close proximity to the
male, he would move. Both of the gibbons would then engage in a physical display,
consisting of vigorously brachiating around the enclosure and shaking substrates or the fence.
The nature of this behavior is not indicative of a cohesive pair-bond.
Considering the gibbons at JGC did not meet the recommended 7 percent of total
activity time engaged in social behavior, but do show slightly similar results to other wild
gibbons (~2-3%) in terms of social behavior (Bartlett, 2003; Ellefson, 1974), I do not think
58
the recommended criteria of positive pair association as proposed necessarily corresponds to
pair-bond cohesiveness for the gibbons at JGC.
STEREOTYPIC BEHAVIOR
The observed pairs of gibbons at JGC did exhibit mild forms of stereotypic behavior.
Most commonly observed was repetitive swinging on a fixed substrate, repetitive
brachiation, and less frequently, teeth scraping. The gibbons would most often engage in
repetitive swinging or brachiation around feeding times and only one of the male gibbons
(Septa) exhibited a mild form of teeth scraping, usually upon my arrival for observations.
Cheyne (2004) observed gibbons at Kalaweit gibbon center actually scraping their canine
teeth against a substrate. This was never observed at JGC. Septa would bite at the fence and
it was typically one or two quick bites. The stereotypic behaviors displayed by the Javan
gibbons at JGC (as well as gibbons at Kalaweit) have never been reported in the wild. This
would seem to suggest that stereotypic behavior could be caused by captivity, or at least
exacerbated by captivity (Cheyne, 2004). Some researchers believe that stereotypic behavior
is a means of coping with a present or past aversive situation (Mason, 1991; Mason &
Berkson, 1975). Stereotypic behaviors become ingrained in the behavioral repertoire of the
animal and are probably the result of poor rearing conditions from when they were kept as
pets, or can develop in any captive animal as the result of a stressful environment (Mason,
1991). Considering each gibbon included in this study exhibited a relatively low level of
harmless stereotypic behavior, it is unlikely these behaviors would persist once they are
released or hinder their survival in any way and should not prevent a pair of gibbons from
being reintroduced.
HUMAN-DIRECTED BEHAVIOR
Human-directed behavior was relatively uncommon in the adult gibbons included in
my study. It was more common for the female gibbons in my study to exhibit human-
directed behavior than the male gibbons. Jeffry had no instances of human-directed behavior
and Sadewa had only one. Both Nancy and Kiki rarely exhibited any human-directed
behavior as well. As part of the criteria for deeming a pair suitable for release, I expected to
observe very little human-directed behavior from the gibbons. It could prove detrimental to
the survivability of the gibbon if it was still tame and sought human attention once released
59
into the forest. None of the gibbons in my study exhibited affiliative behavior towards
humans. Some of the gibbons were less aggressive than others (i.e., not attempting to grab
the keeper), but they did not solicit grooming or petting. Septa and Echi, on the other hand,
had a comparatively high rate of human-directed behavior (Septa: 12 instances/5,962
behavioral records; Echi: 58 instances/5,962 behavioral records). Both gibbons were
extremely aggressive towards the keepers and would attempt to grab them at almost every
feed. In addition, if someone approached their enclosure Echi would typically present a
posterior display or behave in a display like manner by brachiating around the top of the
enclosure and shaking the fence or substrates.
Gibbons that were former pets (i.e., human-raised) can sometimes be aggressive
towards humans upon reaching sexual maturity and tend to direct this aggression at the same
sex individual (Mootnick et al., 2006; Mootnick & Nadler, 1997). The posterior displays
were most commonly displayed by the females in my study, although Septa was observed
displaying this behavior as well. This behavior in gibbons has not been extensively
documented in the literature, except in the case of Mootnick and Nadler (1997). It is
suggested that the posterior display is a signal to conspecifics and could be a result of
misplaced sexual attention, submission followed by fear, or anger followed by aggression
(Cheyne, 2004). The “anger followed by aggression” behavior was the most common
observed response from Echi. If one of the keepers or myself approached her enclosure, she
almost always displayed her posterior, but then would also engage in a physical display (i.e.,
brachiating around the enclosure, shaking substrates or fence). While it was clear that
humans were the focus of this behavior, I never had the opportunity to observe if the gibbons
would also display this behavior to conspecifics. Kiki would present her posterior if humans
approached the enclosure, but she never displayed after presenting. I never observed Nancy
displaying this behavior. One explanation for the aggressive behavior is that the gibbons are
treating humans as potential threats, specifically when the behavior is directed at the same
sex human. Mitani (1984) states that aggressive or territorial behavior in wild gibbons is
almost always intrasexual; males displace males and females displace females. The posterior
display, in addition to other forms of human-directed behavior, is most likely a construct of
captive rearing and of being in continuous close proximity to humans. The levels at which
gibbons in captivity, who are also candidates for release, engage in forms of aggressive
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human-directed behavior should be monitored more closely and can potentially provide some
insight in to how the gibbons will respond to humans once they are released (see discussion
on post-release behavioral observation with regards to human-directed behavior).
LOCATION IN ENCLOSURE
Gibbons are canopy dwellers and specialists at feeding from the terminal branches of
the tallest trees (Cheyne, 2004). They make use of their home range three-dimensionally in
space, as they exploit the canopy upwards and downwards to about ten meters above the
ground (Cheyne, 2004; Kappeler, 1981). Wild gibbons rarely come to the ground due to
higher risk of predation, though captive-raised gibbons are very comfortable spending time
on the ground and will walk on the ground bipedally. Gibbons from the illegal pet trade were
removed from the wild as infants and most likely never had the opportunity to learn valuable
predator avoidance behaviors from their parents. Furthermore, they probably were kept in
small cages relatively low to the ground, if not actually on the ground, hence never being
able to spend time at a higher level (Cheyne, 2004).
The gibbons at JGC came down to the floor of their enclosures to retrieve fallen food
or forage, but rarely will spend more than a few minutes there; typically grabbing the food
and then quickly climbing back up to the higher level of the enclosure. This behavior would
seem to indicate an innate desire for the gibbons to remain within a higher level of space.
However, one of the male gibbons, Sadewa, would often sit on the floor of the enclosure
while resting. Jeffry also appeared to be quite comfortable spending time in the lower levels
of the enclosure. In some of the enclosures at JGC, including Jeffry and Nancy’s, there are
climbing/swinging substrates placed at lower levels. Perhaps it would prove beneficial to
remove all of the substrates from the lower levels of the enclosures in an attempt to
discourage the gibbons from spending time there. The food and water baskets are also placed
at the mid-level of the enclosures (~2-3 m above the ground), but this is primarily because
the keepers need to be able to place food in the baskets and provide fresh water for the
gibbons. If captive gibbons are to avoid predators and parasites once they are released, they
must be encouraged to spend time at the higher levels of their enclosures and must
demonstrate a preferred use of the top of the enclosure. All of the gibbons included in this
study met the recommended 40 percent of time spent at the upper level of their enclosure.
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LOCOMOTION
Fleagle (1976) reported that wild lar gibbons and siamangs cover approximately 60-
75 percent of their territory by brachiation. Gittins (1983) calculated that ten percent of the
total observed active time for agile gibbons was spent traveling and brachiation was used 75
percent of the time, therefore Cheyne et al. (2008) determined that gibbons in a rehabilitation
program should be utilizing brachiation as their main mode of locomotion. Gibbons in
captivity often do not have the space in their enclosures to brachiate for long distances, which
may also explain why some individuals engage in stereotypic forms of repetitive brachiation.
However, in order for rehabilitated gibbons to be suitable for release, they must demonstrate
they are capable of effectively traveling around their enclosure while utilizing natural
locomotive movements as are observed in wild gibbons.
The gibbons at JGC are slightly limited with the size constraints of their enclosures,
but are still capable of acquiring the appropriate forms of locomotion and utilizing them
within their enclosure. All three pairs of gibbons in this study demonstrated their ability to
effectively locomote, and with the exception of Jeffry, utilized brachiation as their primary
mode of locomotion. With regards to stereotypic brachiation, Sadewa and Kiki did utilize
brachiation as their primary form of travel (94% and 87% respectively), though at times it
was difficult to ascertain between simply traveling around the enclosure and when it became
“stereotypic.” Therefore, there could be a discrepancy in the data between the levels of
stereotypic behavior and brachiation displayed by Sadewa and Kiki. However, what is
important is that they are capable of effectively brachiating and if they are going to be
released into the wild, they would be able to successfully traverse through the canopy.
CONSERVATION IMPLICATIONS
The purpose of this thesis was to implement a more systematic and scientific
approach to the reintroduction program at the Javan Gibbon Rescue and Rehabilitation
Center. Cheyne (2004) states that, many reintroduction programs have released animals
based on subjective impression rather than objective scientific data. Until this issue is
addressed and the rehabilitation is carried out under scientifically proven guidelines, many
reintroduced animals will continue to disappear, having contributed nothing to the overall
survival potential of the species. It is my hope that the results from my research will help to
62
inform conservation policies aiming to preserve the Javan gibbon and determine if the
reintroduction of former pet Javan gibbons will serve as a useful conservation tool for
helping prevent their extinction in the wild.
Rehabilitation and reintroduction of endangered species has been heavily criticized
because some argue there is little justification for the continuation of these projects because
they are expensive, have limited success, and tend to focus too narrowly on one species
(Kleiman, 1989). Much of the failure of rehabilitation and reintroduction stems from the lack
of knowledge about the specific requirements of the focal species in terms of their social,
behavioral, and nutritional needs (Cheyne et. al., 2008), which is precisely why having
thorough knowledge of species’ behavior and ecology in the wild is crucial to the
reintroduction process. The conservation value of rehabilitation has historically been
questionable; therefore, it is the goal of this research to become the first step in developing a
more systematic approach to the rehabilitation process at JGC, which will then hopefully
ensure a more successful reintroduction program for Javan gibbons in West Java.
A major challenge for the reintroduction program is locating an appropriate release
site for Javan gibbons on an island where deforestation is one of the primary causes of their
decline. Habitat where gibbons have previously been known to exist, but that are currently
not inhabited by the species, would be the ideal location to release a pair of gibbons.
However, caution must be taken to ensure that the reasons for the local declines in that area
have been addressed. If poaching and/or deforestation were the primary causes of the species
becoming extinct in the area, then evidence must be presented to show that the problem has
been eliminated or drastically reduced (Cheyne et al., 2008; Kleiman, 1989). Extensive
habitat surveys should be conducted in order to determine if the release site will be able to
sustain a pair of gibbons and any future offspring. There has been some criticism regarding
the size of the release site (five hectares) in which Septa and Echi were released and that the
small forest patch will not be able to sustain the pair of gibbons for the long-term. The
release site was chosen specifically because it is small and isolated, and would be easier to
monitor the gibbons post-release. The site is not intended to be the gibbons’ permanent
home range within the national park. A forest corridor has been planted and the trees will
eventually reach heights that the gibbons will then be able to traverse across the corridor and
move out into the larger part of the park. Furthermore, Malone (2007) observed one group of
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Javan gibbons (five individuals) that occupied a home range of approximately 6.25 hectares
in CALS. The gibbons are essentially restricted to this home range because it is bordered by
the coast to the south, a river to the west, a forest clearing to the east, and another group of
gibbons to the north (Malone, 2007), yet the gibbons are able to sustain themselves in this
smaller-than-average home range.
In regards to future release sites for Javan gibbons, the staff of JGC and CI-Indonesia
have identified three potential sites in Gunung Gede-Pangrango National Park for the next
reintroduction: Nagrak (Sukabumi), Cimande (Bogor), and Mandalawangi (Cianjur). The
potential sites are not isolated like Patiwel and are all primary forest. Considering that the
behavioral and ecological data on Javan gibbons are lacking, we must engage in long-term
field studies on the local habitat requirements, dietary needs, and population dynamics of the
species’ in order to best implement a conservation action plan for preserving the species in
the wild. The knowledge obtained from such studies will help to inform conservation
policies regarding demographic data, habitat carrying capacity of potential release sites, and
both inter and intra-species relationships that will be necessary for a successful reintroduction
program (Southwick & Blood, 1979).
EVALUATION OF BEHAVIORAL CRITERIA
Based on Cheyne’s (2004) research conducted at the Kalaweit gibbon center in
Central Kalimantan, where she documented pre-and post-release behavior of rehabilitated
gibbons, she developed a “Behavior Checklist” for gibbons in rehabilitation programs. The
criteria (Cheyne et al., 2008) highlight important aspects of wild gibbon behavior that a
rehabilitated pair of gibbons should master before they are released.
The first recommendation is that the gibbons should be able to move around their
enclosure efficiently and should be utilizing brachiation as their primary mode of travel. All
of the gibbons in this study, except one, satisfied this requirement. Considering it is crucial
that gibbons in the wild are able to effectively traverse through the forest using brachiation as
their main form of travel, I would say this is a necessary criterion that gibbons in the
rehabilitation program should meet if they are to be considered for release.
It is suggested that gibbons should spend no more than five percent of their time on
the ground level of their enclosure, and they should be at the upper level of their enclosure at
64
least 40 percent of the time. All of the Javan gibbons in this study met this requirement.
Once released, if the gibbons are to avoid risk of predation and survive in the wild, they must
remain in the upper level of the canopy. Former pet gibbons may be very comfortable
spending time on the ground and if this behavior is present in gibbons in the rehabilitation
program, it must be eliminated prior to release. This requirement is a necessary part of the
rehabilitation procedure.
Cheyne et al. (2008) recommends that a pair of gibbons should be spending at least
seven percent of their time engaged in positive pair association (i.e., groom, play,
copulation), with at least three percent of total activity time spent allogrooming. None of the
pairs of gibbons in this study satisfied these criteria. However, the criteria as proposed do
not acknowledge that one of the primary forms of social interaction for adult gibbons is
“sitting together” within close proximity (within one meter) (Palombit, 1994), thus it is not
included in positive association behavior. If the maintenance of close proximity were to be
included in positive association behavior, all three of the pairs of gibbons in this study would
have met the suggested requirement of at least seven percent. Jeffry and Nancy spent 17.6
percent of observation records in close proximity, Sadewa and Kiki spent 12.6 percent of
observation records in close proximity, and Septa and Echi had the highest percent of
observation records spent in close proximity at 45.7 percent. I would suggest including the
maintenance of close proximity in the positive association behavior category as part of the
necessary criteria for rehabilitated gibbons.
The guidelines also state that the pair of gibbons should be copulating successfully
and each individual should be able to initiate copulation with the other. Only one pair
included in this study was observed copulating (Septa and Echi), however, that does not
mean the other two pairs are not copulating. The breeding pair of Javan gibbons at the Perth
Zoo has only been observed copulating two times (H. Thompson, personal communication,
October 7, 2009). Therefore, it is possible that copulation between a pair of gibbons is
simply not being observed. If a pair of gibbons is not observed copulating by human
observers, it may prove beneficial to set up a video camera system to attempt to observe the
gibbons copulating. If a pair of gibbons are housed together for several years and do not
produce any offspring, it may prove beneficial for conservation efforts to re-pair the gibbons
with other individuals so that they may have the opportunity to successfully reproduce. It is
65
necessary that gibbons in a rehabilitation program are observed copulating, via human
observation or video camera, prior to their release, so that they do contribute to the wild
population.
In general, the gibbons in this study showed similar patterns of behavior with regards
to the activity budgets of wild Javan gibbons. The ways in which they differ were most
likely due to the captive environment. For example, the gibbons in this study spent more
time resting and less time feeding/foraging than their wild counterparts. This is likely
because gibbons in the wild do have to spend more time traveling to locate food sources. I
would expect to see a higher percent of resting in captive gibbons because they do not have
the space to travel farther distances and they have their food provisioned. Ideally, gibbons in
a rehabilitation program should exhibit similar behavioral patterns to their wild conspecifics,
while at the same time we must acknowledge the limitations placed on them as a result of a
captive environment. Therefore, I do not include this criteria in the amended behavioral
checklist.
The guidelines state that gibbons in a rehabilitation program should not be exhibiting
severe stereotypic behavior (i.e., rocking or self harm) more than three percent of total
activity time. Considering the nature of the stereotypic behavior displayed by the gibbons in
this study was not severe, I considered all three of the pairs to have satisfied this criterion.
The primary stereotypic behavior observed was repetitive brachiation or swinging on a
substrate, and it is unlikely that when the gibbons are released these behaviors will persist;
these behaviors are the result of a captive environment. However, stereotypic behavior
should be monitored during the rehabilitation program and depending on the nature of the
behavior, the decision to deem a gibbon suitable for release can be assessed based on the
severity of the behavior. I recommend that if a gibbon exhibits severe stereotypic behavior,
that it not be released into the wild. Perhaps the individual would be more beneficial
remaining at JGC and used for a captive breeding program, and then any future offspring
may be released into the wild.
I would also recommend adding a diet category to the behavioral criteria. The Javan
gibbons at JGC have fruit and foliage that is collected from the forest in Gunung Gede-
Pangrango incorporated in to their diet. This is done so that the gibbons will be accustomed
to eating a more natural diet. It is important that any gibbon who is a candidate for
66
reintroduction must show a preference for wild fruit and foliage before they are released.
They must enjoy eating these sources of food and become familiar with them visually, so that
when they are living in the wild, they are able to identify them in the canopy.
Overall, the behavioral checklist is an effective tool for determining the preparedness
of a pair of gibbons prior to reintroduction, but based on my data could be amended. The
rehabilitation and reintroduction of gibbons is a relatively new conservation strategy and
there are very little data published on the successful reintroduction of gibbons back into the
wild. With 15 of the 17 gibbon species currently listed as Endangered or Critically
Endangered (IUCN, 2010; Van Ngoc et al., 2010), the future of all gibbons remains uncertain
(Cheyne et al., 2008). The reintroduction of former pet gibbons may play a significant role
in the preservation of many of these threatened species of gibbons. However, it is crucial
that the rehabilitation procedure is fully documented. This is the only way to establish a
well-rounded rehabilitation/reintroduction program that adheres to scientific guidelines and
principles.
Table 5 discusses the revisions/additions I have made to Cheyne et al.’s (2008)
behavioral checklist based on the results from this study.
LESSONS LEARNED FROM THE RELEASE
When I arrived at JGC to conduct my field work in July 2009, I was informed that the
first pair of Javan gibbons (Figure 15) was going to be released during the month of October
2009. As a result, I was presented with the opportunity to conduct behavioral observations
on the pair before and after they were released. The pair was moved to an acclimation
enclosure at the release site, Patiwel, a month and a half before they were released into the
forest. Therefore, I was able to compare their behavior at JGC and at Patiwel pre-release,
and also their behavioral response to the forest once they were released.
Due to a relatively high number of Javan gibbons that are currently held illegally in
Indonesia as pets, rescue and reintroduction may be an effective conservation strategy for
reestablishing populations of Javan gibbons in the wild. However, based on Septa and Echi’s
response to human observers after they were released, precaution will need to be taken for
future releases when monitoring reintroduced gibbons. I expected to observe very little
human-directed behavior, either affiliative (i.e., soliciting grooming, petting) or aggressive
67
Table 5. Revised Behavioral Checklist
Proposed criteria for assessing suitability of gibbons for release.*
Behavioral recommendations based on the results obtained from this thesis.
The gibbon should be able to move around the enclosure well, and most of this movement should be by brachiation
Gibbons should be able to effectively locomote and should utilize brachiation as their primary mode of travel around the enclosure.
No more than 5% of time to be spent on the ground for any purpose. Gibbons should be at the top of the cage for at least 40% of the time and should not be sleeping on the ground at all.
Gibbons should be spending the majority of their time at the upper level of their enclosure, with no more than 5% of time spent on the ground.
The pair should be spending at least 7% of total activity time in positive pair association. At least 3% of total activity time should be spent allogrooming.
Gibbons should be fed a diet that includes wild fruits and foliage, and they must demonstrate a significant interest in these foods.
They should be copulating successfully and each member of the pair should be able to initiate successful copulation with the other.
The pair should be spending at least 15% of total observation records in positive association behavior (i.e., groom, play, copulation, and within close proximity).
Activity budgets should approximate those of wild conspecifics in all major categories i.e., feeding, resting and travelling.
They should be copulating successfully and each member of the pair should be able to initiate successful copulation with the other.
No more than 3% of total activity time engaged in a severe stereotypic behavior e.g., rocking or self-harm.
Gibbons should not exhibit any form of severe stereotypic behavior and very infrequent occurrences of human-directed behavior.
* Cheyne, S. M., Chivers, D. J., & Sugardjito, J. (2008). Biology and behavior of reintroduced gibbons. Biodiversity Conservation, 17, 1741-1751.
68
Figure 15. Septa and Echi post-release (Photo by Anton Ario/CI-Indonesia).
behavior, once the gibbons were released. This expectation is based on the finding that wild
gibbons tend to shy away from humans and flee when they encounter them in the forest, but
will sometimes perform threat displays or alarm calls (I. Supian, personal communication,
October 24, 2009; Malone, 2007).
Captive gibbons, on the other hand, may have very little fear of humans. There has
never been a published report of a wild gibbon physically attacking a human. Although
Septa and Echi were highly aggressive towards humans while in captivity, I did not expect
the aggressive behavior would persist once they were released. There is always a potential
risk in releasing animals into the wild that have lived in captivity with humans for most of
their life, because they will either have developed a psychological attachment or resentment
towards humans, or will have no fear of them. There are reports of reintroduced orangutans,
bonobos, and gibbons attacking humans (Les Amis des Bonobo du Congo Newsletter, 2009;
69
The Wild Animal Rescue Foundation of Thailand, 2010; Yeager, 1997). For example, one
adolescent male orangutan attacked and bit humans over 25 times in a 15 month period in
Tanjung Puting National Park in Indonesia (Yeager, 1997). In Phuket, Thailand at the
Gibbon Rehabilitation Project (GRP), aggression directed at humans was displayed by two
adult lar gibbons immediately after they were released from their enclosure. When the staff
would attempt to observe the gibbons in the forest, the male would approach observers
jumping from tree to tree at a low height in the canopy or running bipedally on the ground.
Some of the observers, including both volunteers and staff, were actually scratched or bitten
by the male gibbon. The female (and the juvenile) would sometimes urinate or defecate on
humans or their belongings. She also scratched some observers and seemed to specifically
dislike female observers (The Wild Animal Rescue Foundation of Thailand, 2010). At JGC,
Echi showed a similar dislike for me while she was in captivity. Once she was released, she
directed aggressive behavior at all human observers (I was the only female observer). The
staff of GRP would use sling shots to attempt to scare the gibbons and the gibbons eventually
learned to back off when they became aware of the sling shots. We also used sling shots in
an attempt to frighten Septa and Echi back up into the canopy, however, this method did not
work for us. GRP reports that the aggressive behavior of the adult male towards observers
eventually decreased during the study period, while the adult female still exhibited
aggressive behavior to every new female observer. The gibbons at Kalaweit were released
onto an island (Mintin) and most of the post-release monitoring was conducted from a boat
off shore. However, observers would occasionally go onto the island and attempt to locate
the gibbons, but there were never any aggressive encounters between the gibbons and human
observers.
Considering Septa and Echi were removed from the illegal pet trade and have spent
the majority of their life in captivity with human interaction prior to being released, it is very
likely that they may maintain some degree of abnormal behavior. However, even though the
aggressive behavior they displayed towards humans is not typical for wild gibbons, this does
not mean they cannot survive in the wild. It may actually be advantageous for them when
they are able to move into the larger part of the park and have to establish a new territory
near wild gibbons. They have demonstrated that they are capable of effectively defending
their territory and so they will hopefully be able to sustain themselves in the larger
70
population of wild gibbons in Gunung Gede-Pangrango National Park. Septa and Echi
maintained normal species typical behavior with one another and their bond remained intact.
After the gibbons at Kalaweit were released onto Mintin Island, the association between the
pair broke down and there was no longer any social interaction or singing. The pair of
gibbons did come back together after four weeks (Cheyne, 2004). In addition to engaging in
normal social behavior with one another, Septa and Echi made use of all substrates available
to them in the forest, despite never having any experience prior to being released that would
have prepared them for the complexities of the forest. Ultimately, it would appear that Septa
and Echi have developed appropriate survival skills and demonstrate that some gibbons are
capable of some level of rehabilitation after having spent the majority of their life in
captivity. For future releases, it will be critical to ensure the release site has very little human
activity in the area in order to minimize the encounter rate between gibbons and humans.
Each gibbon has a different life history and the outcome of future release efforts will likely
vary on a case by case basis.
LOCAL INITIATIVES AND SUPPORT
Support from local governments and communities is crucial and one way to ensure a
successful reintroduction program for Javan gibbons in West Java. Based on a
comprehensive review of previous research and an assessment of continued threats to the
Javan gibbon and its habitat, Supriatna (2006) concluded that educational outreach and
public support, as well as participation, will be invaluable components in the elimination of
existing pressures and the implementation of conservation initiatives (Malone, 2007).
Conservation programs should devote sufficient time and energy into the development of
education programs that will suit all audiences and focus on all aspects of conservation
(Nijman, 2004). In order to increase community awareness and knowledge on biodiversity
conservation in Gunung Gede-Pangrango National Park, CI-Indonesia has enlisted the use of
a mobile conservation education unit that travels to local communities teaching conservation
education. The primary goal of the conservation education program is to encourage local
people to incorporate conservation concepts into their daily activities and provide them with
more knowledge regarding the local flora and fauna. For children, the unit uses “Moli” the
Javan gibbon and “Telsi” the Javan hawk-eagle as characters to deliver the program’s
71
conservation message. In addition, the Mobile Conservation Education Unit has a small
library that is accessible to local communities.
Furthermore, regarding the primary issues of habitat loss and the illegal trade in
gibbons, local people will have to be involved in the long-term effective management of
forest resources and they must demonstrate the will to reject, report, and prosecute
individuals who are involved in the illegal pet trade (Malone, 2007). The local people are not
provided with adequate information regarding the importance of conservation and the long
term benefits they can derive from the forests (i.e., watershed protection) (Supriatna, 2006).
It is crucial for the Indonesian government, NGOs, scientists, and other concerned
stakeholders to merge their efforts regarding the best conservation policies that will benefit
the indigenous people of Java and its wildlife (Orlove & Brush, 1996). Conservation is about
changing the way in which people perceive the planet, including the wildlife with which they
share it with. It is imperative to establish a well-rounded conservation education program
that emphasizes the value of all wildlife and its role in maintaining forest biodiversity.
The purpose of any reintroduction program should be thoughtfully weighed against
the costs and benefits of alternative conservation strategies, including protection of the wild
population. Rehabilitation and reintroduction of former pet gibbons can potentially serve to
make a considerable contribution to the reestablishment of viable wild populations, as well as
reduce significant welfare issues for gibbons already living in rescue centers (Nijman, 2006).
It has been suggested that conservation efforts should focus on preserving the remaining
habitat and populations of wild primates and that the bulk of the available funding should be
redirected to these causes (Sarrazin & Barbault, 1996). However, if rehabilitation and
reintroduction programs adhere to scientific principles and guidelines, coupled with careful
planning and documentation of the pre- and post-release behavior of the animals, the
programs can positively contribute to species’ conservation and habitat protection (Cheyne,
2009). Ultimately, a reintroduction program cannot succeed without the researchers
interacting with local and national governments, government and non-government
professionals, and the local people in the communities. Education about the reintroduction
program is crucial to secure continuity and the long-term support, protection, and
management of the Javan gibbon and its habitat and ensure that this fascinating ape does not
go extinct on the island of Java (Kleiman, 1989).
72
REFERENCES
Altmann, J. (1974). Observational study of behavior: Sampling methods. Behaviour, 49, 227-267.
Andayani, N., Morales, J. C., Forstner, M. R. J., Supriatna, J., & Melnick, D. J. (2001). Genetic variability in mtDNA of the silvery gibbon: Implications for the conservation of a critically endangered species. Conservation Biology, 15, 770-775.
Asquith, N. M. (1995). Javan gibbon conservation: Why habitat protection is crucial. Tropical Biodiversity, 3, 63-65.
Atkinson, M. W. (1997). New perspectives on wildlife rehabilitation. Zoo Biology, 16, 355-357.
Ballou, J. D, Kleiman, D. G., Mallinson, J. J. C., Rylands, A. B., Valladares-Padua, C. B., & Leus, K.(2002). History, management, and conservation role of the captive lion tamarin populations. In D. G. Kleiman & A. B. Rylands (Eds.), Lion tamarins: Biology and conservation (pp. 95-114). Washington, DC: Smithsonian Institution Press.
Bartlett, T. Q. (2003). Intragroup and intergroup social interactions in white-handed gibbons. International Journal of Primatology, 24, 239-259.
Bartlett, T. Q. (2007). The hylobatidae: Small apes of Asia. In C. J. Campbell, A. Fuentes, K. C. MacKinnon, M. Panger, & S. K. Bearderm (Eds.), Primates in perspective (pp. 274-289). New York, NY: Oxford University Press.
Britt, A., Welch, C., & Katz, A. (2003). Can small, isolated primate populations be effectively reinforced through the release of individuals from a captive population? Biological Conservation, 115, 319-327.
Brockelman, W. Y. (1984). Social behaviour of gibbons. In H. Preuschoft, D. J. Chivers, W. Y. Brockelman, & N. Creel (Eds.), The lesser apes: Evolutionary and behavioural biology (pp. 285-290). Edinburgh, Scotland: Edinburgh University Press.
Brockelman, W. Y., Reichard, U., Treesucon, U., & Raemaekers, J. J. (1998). Dispersal, pair formation, and social structure in gibbons (Hylobates lar). Behavioral Ecology and Sociobiology, 42, 329-339.
Chapman, C. A. (1995). Primate seed dispersal: Coevolution and conservation implications. Evolutionary Anthropology, 4, 74-82.
Cheyne, S. M. (2004). Assessing rehabilitation and reintroduction of captive-raised gibbons in Indonesia. (Doctoral dissertation). University of Cambridge, Cambridge, England.
Cheyne, S. M. (2006). Wildlife reintroductions: Consideration of habitat quality at the release site. BMC Ecology, 6, 1-8.
Cheyne, S. M. (2009). Challenges and opportunities of primate rehabilitation-gibbons as a case study. Endangered Species Research, 9, 159-165.
73
Cheyne, S. M., Chivers, D. J., & Sugardjito, J. (2008). Biology and behavior of reintroduced gibbons. Biodiversity Conservation, 17, 1741-1751.
Chivers, D. J. (1978). Sexual behaviour of wild siamang. In D. J. Chivers & J. Herbert (Eds.), Recent advances in primatology (pp. 609-610). London, England: Academic Press.
Clemmons, J. R., & Buchholz, R. (1997). Linking conservation and behavior. In J. R. Clemmons & R. Buchholz (Eds.), Behavioral approaches to conservation in the wild (pp. 3-22).Cambridge, England: Cambridge University Press.
Cowlishaw, G., & Dunbar, R. (2000). Primate conservation biology. Chicago, IL: The University of Chicago Press.
de Veer, M. W., & van den Bos, R. (2000). Assessing the quality of relationships in rehabilitating lar gibbons (Hylobates lar). Animal Welfare, 9, 223-224.
Ellefson, J. O. (1974). A natural history of white-handed gibbons in Malayan peninsula. Gibbon and Siamang, 3, 1-136.
Fleagle, J. G. (1976). Locomotion and posture of the Malayan siamang and implications for hominid evolution. Folia Primatologica, 26, 245–269.
Fleagle, J. G. (1988). Primate adaptation and evolution. New York, NY: Academic Press.
Froehlich J. W, Supriatna, J., & Muskita, Y. (1996). Biodiversity and the conservation of Macaca nigra: An anthropological view. Tropical Biodiversity, 3, 85–102.
Fuentes, A. (1999). Re-evaluating primate monogamy. American Anthropologist, 100, 890-907.
Fuentes, A. (2000). Hylobatid communities: Changing views on pair-bonding and social organization in hominoids. Yearbook of Physical Anthropology, 43, 33-60.
Geissmann, T. (2002). Duet-splitting and the evolution of gibbon songs. Biological Review, 77, 57-76.
Geissmann, T., Dallmann, R., & Pastorini, J. (2002). The Javan silvery gibbon (Hylobates moloch): Are there several subspecies? Retrieved October 14, 2010, from http://www.gibbons.de/main/congress/02beijing3silvery.html
Geissmann, T., & Nijman, V. (2006). Calling in wild silvery gibbons (Hylobates moloch) in Java (Indonesia): Behavior, phylogeny, and conservation. American Journal of Primatology, 68, 1-19.
Gittins, S. P. (1983). Use of the forest canopy by the agile gibbon. Folia Primatologica, 40, 134-144.
Gittins, S. P., & Raemakers, J. J. (1980). Siamang, agile and lar gibbons. In D. J. Chivers (Ed.), Malayan forest primates: Ten years study in tropical rainforest (pp. 258-266). New York, NY: Plenum Press.
Groves, C. P. (1984). A new look at the taxonomy and phylogeny of the gibbons. In H. Preuschoft, D. J. Chivers, W. Brockelman, & N. Creel (Eds.), The lesser apes: Evolutionary and behavioural biology (pp. 542-561). Edinburgh, Scotland: Edinburgh University Press.
74
Groves, C. P. (2001). Primate taxonomy. Washington, DC: Smithsonian Press.
Grundman, E. (2006). Back to the wild: will reintroduction and rehabilitation help the long-term conservation of orangutans in Indonesia? Social Science Information, 45, 265-284.
Harlow, H. F., & Suomi, S. J. (1971). Social recovery by isolation-reared monkeys. Proceedings of the National Academy of Sciences USA, 68, 1534-1538.
Hidayat, H. (2008). Forest management policy Indonesia. In N. Kusumawardani & B. Wahyunarso (Eds.), Politik lingkungan: Pengelolaan hutan masa Orde Baru dan reformasi (pp. 114-122). Jakarta, Indonesia: Yayasan Obor Indonesia.
Hodgkiss, S., Thetford, E., Waitt, C. D., & Nijman, V. (2009). Female reproductive parameters in the Javan gibbon (Hylobates moloch). Zoo Biology, 28, 1-8.
International Union for Conservation of Nature and Natural Resources [IUCN]. (2002). Guidelines for nonhuman primate reintroductions. Reintroduction News, 21, 1-32.
International Union for Conservation of Nature and Natural Resources [IUCN]. (2010). IUCN Red list of threatened species. Version 2010.4. Retrieved October 14, 2010, from http://www.iucnredlist.org
Kalaweit Gibbon Center [KP]. (2010). Homepage. Retrieved October 14, 2010, from http://www.kalaweit.org
Kappeler, M. (1981). The Javan silvery gibbon (Hylobates lar moloch). (Doctoral dissertation) Universitat Basel, Basel, Switzerland.
Kim,S., Lappan, S., & Choe, J. C. C. (2010). Diet and ranging behavior of the endangered Javan gibbon (Hylobates moloch) in a submontane tropical rainforest. American Journal of Primatology, 71, 1-10.
Kleiman, D. G. (1989). Reintroduction of captive mammals for conservation. BioScience, 39, 151- 163.
Komdeur, J., & Deerenberg, C. (1997). The importance of social behavior studies for conservation: In J. R. Clemmons & R. Buchholz (Eds.), Behavioral approaches to conservation in the wild (pp. 262-276). Cambridge, England: Cambridge University Press.
Leighton, D. S. R. (1987). Gibbons: Territoriality and monogamy. In B. B. Smuts, D. L. Cheney, R. M. Seyfarth, R. W. Wrangham, & T. T. Struhsaker (Eds.), Primate societies (pp. 135-145). Chicago, IL: University of Chicago Press.
Les Amis des Bonobos du Congo Newsletter. (2009). Ekolo ya bonobo: Bonobo Reintroduction in the Democratic Republic of Congo. Retrieved October 14, 2010, from http://www.friendsofbonobos.org/
MacKinnon, J. (1977). A comparative ecology of Asian apes. Primates, 18, 747-722.
Malone, N. M. (2007). The socioecology of the critically endangered Javan gibbon (Hylobates moloch); Assessing the impact of anthropogenic disturbance on primate social systems. (Doctoral dissertation) University of Oregon, Portland, OR.
75
Malone, N. M., Fuentes, A., Purnama, A. R., & Adi Putra, I. M. W. (2004). Displaced hylobatids: Biological, cultural, and economic aspects of the primate trade in Java and Bali, Indonesia. Tropical Biodiversity, 40(1), 41-49.
Marshall, A. J. (2009). Are montane forests demographic sinks for Bornean white-bearded gibbons (Hylobates albibarbis). Biotropica, 41, 257-267.
Mason, G. J. (1991). Stereotypes: A critical review. Animal Behaviour, 41, 1015-1037.
Mason, W. A., & Berkson, G. (1975). Effects of maternal mobility on the development of rocking and other behaviours in rhesus monkeys: a study with artificial mothers. Developmental Psychobiology, 8, 197-211.
McConkey, K. R. (2000). Primary seed shadow generated by gibbons in the rain forest of Barito Ulu, central Borneo. American Journal of Primatology, 52, 13-29.
Mitani, J. (1984). The behavioral regulation of monogamy in gibbons (Hylobates muelleri). Behavior Ecology and Sociobiology, 15, 225-229.
Mittermeier, R. A., Gil, P. R., Hoffman, M., Pilgrim, J., Brooks, T., Goettch Mittermeier, C.,…da Fonseca, A. B. (2005). Hotspots revisited: Earth's biologically richest and most endangered terrestrial ecoregions. Chicago, IL: University of Chicago Press.
Mootnick, A. R. (2006). Gibbon (Hylobatidae) species identification recommended for rescue or breeding centers. Primate Conservation, 21, 103-138.
Mootnick, A. R., Baker, E., Nadler, R. D., & Merker, B. (2006). Hostile presenting in captive gibbons. International Journal of Primatology, 27, 809-825.
Mootnick, A. R., & Groves, C. P. (2005). A new generic name for the hoolock gibbon (Hylobatidae). International Journal of Primatology, 26, 971-976.
Mootnick, A. R., & Nadler, R. D. (1997). Sexual behavior of maternally separated gibbons (Hylobates Spp). Developmental Psychobiology, 2, 149-161.
Nijman, V. (2004). Conservation of the Javan Gibbon (Hylobates moloch): Population estimates, local extinctions, and conservation priorities. The Raffles of Bulletin of Zoology, 52, 271-280.
Nijman, V. (2006). In-situ and ex-situ status of the Javan gibbon and the role of zoos in the conservation of the species. Contributions to Zoology, 75, 161-168.
Nijman, V., & van Balen, S. (1998). A faunal survey of the Dieng Mountains,Central Java, Indonesia: Distribution and conservation of endemic species. Oryx, 32, 145-156.
Nijman, V., Yang Martinez, C. –F., & Shepherd, C. R. (2009). Saved from trade: donated and confiscated gibbons in zoos and rescue centres in Indonesia. Endangered Species Research, 9, 151-157.
Orlove, B. S., & Brush, S. B. (1996). Anthropology and the conservation of biodiversity. Annual Review of Anthropology, 25, 329-352.
Palombit, R. (1994). Dynamic pair bonds in Hylobatids: implications regarding monogamous social systems. Behaviour, 128, 65-101.
76
Peluso, N. (1993). “Traditions” of forest control in Java: Implications for social forestry and sustainability. Global Ecology and Biogeography Letters, 3, 138-157.
Reichard, U. (1995). Extra-pair copulations in a monogamous gibbon (Hylobates lar). Ethology, 100, 99-112.
Riley, E. P. (2010). The endemic seven: Four decades of research on the Sulawesi macaques. Evolutionary Anthropology, 19, 22-36.
Roos, C., & Geissmann, T. (2001). Molecular phylogeny of the major hylobatid divisions. Molecular Phylogentics & Evolution, 19, 486-494.
Sarrazin, F., & Barbault, R. (1996). Reintroduction: challenges and lessons for basic ecology. TREE, 11, 474-478.
Schoene, C. U. R., & Brend, S. A. (2002). Primate sanctuaries - A delicate conservation approach. South African Journal of Wildlife Research, 32, 109-113.
Siscawati, M. (2010). Forest policy reform in Indonesia: Has it addressed the underlying causes of deforestation and forest degradation? Retrieved October, 14, 2010, from http://enviroscope.iges.or.jp
Smiet, A. C. (1992). Forest ecology on Java: Human impact and vegetation of montane forest. Journal of Tropical Ecology, 8, 129-152.
Smith, H., Chiszar, D., & Montanucci, R. (1997). Subspecies and classification. Herpetological Review, 28, 13-16.
Southwick, C. H., & Blood, B. D. (1979). Conservation and management of wild primate populations. BioScience, 29, 233-237.
Srikosamatara, S., & Brockelman, W. Y. (1987). Polygyny in a group of pileated gibbons via a familial route. International Journal of Primatology, 8, 389-393.
Stanford, C. B. (2001). The subspecies concept in primatology: The case of mountain gorillas. Primates, 42, 309-318.
Stoinski, T. S., Beck, B. B., Bloomsmith, M. A., & Maple, T. L. (2003). A behavioral comparison of captive-born, reintroduced golden lion tamarins and their wild-born offspring. Behaviour, 140, 137-160.
Supriatna, J. (2006). Conservation programs for the endangered Javan gibbon. Primate Conservation, 21, 155-162.
Supriatna, J., Mootnick, A. R., & Andayani, N. (2010). Javan gibbon (Hylobates moloch): Population and conservation. In S. Gursky-Doyen & J. Supriatna (Eds.), Indonesian primates. Series Developments in primatology: Progress and prospects (pp. 57-72). New York, NY: Springer.
The Wild Animal Rescue Foundation of Thailand. (2010). Gibbon rehabilitation project. Retrieved November 18, 2010, from http://www.gibbonproject.org
Tingpalapong, M., Watson, W. T., Whitmire, R. E., Chapple, F. E., & Marshal, J. T. Jr. (1981). Reactions of captive gibbons to natural habitat and wild conspecifics after release. Natural History Bulletin of the Siam Society, 29, 31-40.
77
Van Ngoc, T., Mootnick, A. R., Geissman, T., Ming, L., Ziegler, T., Agil, M., et al. (2010). Mitochondrial evidence for multiple radiations in the evolutionary history of small apes. BMC Evolutionary Biology, 74, 1-13.
Whitten, A. (1982). Home range use by Kloss gibbons (Hylobates klossii) on Siberut Island, Indonesia. Animal Behavior, 30, 182-198.
Whitten, A. (1984). Ecological comparisons between Kloss gibbons and other small gibbons. In H. Preuschoft, D. Chivers, W. Brockelman, & N. Creel (Eds.), The lesser apes. Evolutionary and behavioural biology (pp. 354-375). Edinburgh, Scotland: Edinburgh University Press.
Yeager, C. P. (1997). Orangutan rehabilitation in Tanjung Puting National Park, Indonesia. Conservation Biology, 11, 802-805.